Which smart sensors catch squatters in vacant property?

No consumer sensor detects “a squatter” in the way people usually mean it. A squatter is a person staying in a property, not a single event. The sensor stack has to break that concern into observable signals: a door opened, something moved, a person remained still, air quality patterns changed, or a camera confirmed what the other sensors implied.

That distinction matters most in vacant houses, where the alert usually lands on someone who is not nearby. A clean “front door opened” notification is useful. A vague “motion detected” at 11:42 p.m. is only useful if the person receiving it knows what kind of sensor sent it, what tends to fool it, and whether there is a verification layer before anyone drives across town.

What each sensor actually catches

The cheap layer is entry. The more interesting layer is habitation. The mistake is buying one device advertised as “presence” or “security” and assuming it covers both.

Start with entry, because doors and windows are the cleanest signal

For a vacant property, door and window contacts are usually the first sensors to install because they answer the simplest question: did this opening change state? They do not infer intent. They do not decide whether the person is a contractor, a relative, a burglar, or a future squatter. They just report that an entry point opened.

That narrowness is a feature. A contact sensor on the main door, back door, basement door, garage entry, and accessible windows gives you a map of entry events before you start guessing about movement inside. Aqara’s Door and Window Sensor, for example, is sold with an approximately two-year battery-life estimate and can trigger a hub alarm and push notification when the protected opening is opened.[1]

The failure mode is just as clear: a contact sensor does not know whether someone is sleeping in a back bedroom. It also does not protect an opening you forgot, a broken pane that never separates the magnet, or a door left ajar before the system was armed. If the property has a dog door, cellar hatch, detached garage, or side window hidden by shrubs, those openings have to be treated as real paths, not afterthoughts.

This is where many vacant-property setups either become useful or become noisy. Put contacts on the openings that matter first. Then decide whether the next layer should look for motion, still presence, or longer-term habitation.

PIR motion is not presence

A passive infrared sensor watches for changes in infrared energy across its detection zones. In plain terms, it is good at noticing a warm body moving across its view. It is not good at proving that a person is still in the room.

That is the core problem with using PIR motion as a squatter sensor. A person walking through the hallway may trigger it. A person sitting still in a corner may not. Reconeyez’s guidance on motion-sensor false alarms describes PIR as detecting movement rather than presence and notes that it can miss stationary people; it also lists common false-trigger sources including direct sunlight, HVAC vents, drafts, pets, and insects.[2]

In an occupied home, a false motion alert is annoying. In a vacant home, it changes behavior. Someone may call a neighbor, wake up a family member, or drive to a dark property because a vent moved warm air across a sensor. PIR can still be valuable, but only when the alert means “movement happened in this covered zone,” not “someone is living there.”

Placement matters more than the packaging suggests. PIR sensors are commonly recommended around 2.1 to 2.4 meters, or 7 to 8 feet, and away from windows and vents; pet-immune claims commonly cover animals around 30 to 60 pounds, but that does not make the sensor immune to every animal, insect, heat source, or bad angle.[2]

For a vacant house, PIR belongs in places where movement itself is meaningful: a stairwell, interior hallway, garage-to-house passage, or room that should not have traffic after a contact sensor reports entry. It is weaker in rooms where a person could sit, sleep, or stay mostly still.

mmWave is the consumer layer that can see still presence better

Millimeter-wave presence sensors change the vacant-property conversation because they are not just looking for someone crossing a PIR field. They can detect small human micro-movements, including breathing-level motion, which is why they are better suited to static occupancy than ordinary PIR motion sensors.[3]

This is the part of the category I actually like, with one warning attached: verify the sensing method. Some products use “presence” loosely. For a vacant property, the difference between PIR labeled as presence and actual mmWave presence is not semantic. It decides whether a seated, still person is likely to remain visible to the system.

The tradeoff is installation discipline. A wired Aqara FP2 is specified to cover up to 40 square meters and support up to 30 zones, which is useful in a large room where you want to distinguish a sofa area from an entry path.[4] But that kind of sensor has to be powered and placed correctly. It is not the same installation as sticking a coin-cell contact sensor on a door frame.

Battery-hybrid mmWave devices exist, but their estimates should be read as estimates. Homey’s mmWave guide lists the Aqara FP300 at about three years on Zigbee and about two years on Matter-over-Thread, while also listing the Meross MS605 at up to three years; SwitchBot describes its mmWave Presence Sensor as using two AAA batteries with up to two years of battery life.[3][5]

Those figures are useful for comparison, not as promises. Reporting frequency, temperature, radio conditions, hub distance, and automation design all matter. A vacant house with weak mesh coverage, cold rooms, and frequent wake-ups is not the same environment as a lab-friendly product page.

Use mmWave where static human presence matters: a living room, bedroom, finished basement, or interior area where someone could remain after entry. Do not expect one ceiling or wall sensor to magically classify an entire awkward property. Old houses have alcoves, thick walls, mirrors, fireplaces, and furniture that make neat coverage diagrams less neat.

If privacy is the concern with presence sensing, mmWave has a different risk profile than cameras because it is not producing images. The tradeoffs are still real: room-level behavior can be sensitive, and sensor placement can reveal patterns. A deeper treatment of that distinction belongs in a dedicated mmWave privacy discussion, but for vacant-property selection the immediate question is simpler: do you need to detect a still human, and can you power and place the sensor well enough to trust the alert?

For more on the privacy side of this sensing method, see mmWave presence sensor privacy.

CO2 and occupancy analytics are a different class of tool

CO2-based occupancy analytics is closer to “is this space being inhabited?” than a door contact or PIR sensor, but most of the current squatter-focused positioning lives in commercial property-tech, not normal DIY smart-home hardware.

Alertify’s squatter-detection product describes using sustained CO2 rise and anonymous device counts as signals of unauthorized occupancy, with privacy-safe positioning because it does not rely on audio or video; it also describes tamper detection and signed reports.[6] Occuspace similarly frames occupancy sensing around non-camera approaches such as environmental and device-based signals, with privacy as a major reason to avoid image-based counting in some settings.[7]

That does not make a CO2 monitor on a kitchen counter a consumer squatter detector. CO2 changes need context: ventilation, room volume, HVAC schedules, sensor placement, outdoor air exchange, and time. A short spike is not the same thing as habitation. A commercial analytics platform may combine signals, reporting, tamper status, and portfolio workflows. A DIY air-quality sensor usually does not.

Dwellwell’s commercial announcement describes an AI-driven sensor technology intended to protect real estate against unauthorized occupancy, which again puts this layer in a property-management and analytics category rather than a simple consumer accessory category.[8]

For an individual vacant house, CO2 analytics may be worth looking at if the property is part of a larger managed portfolio, if camera use is constrained, or if the goal is documented occupancy pattern reporting. For a typical owner trying to monitor an inherited house, it is usually not the first purchase.

Cameras verify; they should not be your only logic

A camera is the layer that tells you whether the contact alert was a contractor opening the back door, a raccoon in the garage, a branch moving near a window, or a person carrying bedding through the hallway. That makes it valuable. It does not make it the cleanest primary detector.

Cameras bring their own dependencies: power, Wi-Fi or local recording, lighting or infrared performance, field of view, storage, notification settings, privacy limits, and sometimes subscription rules. In a vacant property, they work best as verification after a lower-power sensor raises a specific event.

A practical sequence looks like this: contact sensor reports the back door opened; an interior PIR or mmWave sensor reports movement or presence in the hallway; a camera clip confirms whether a person is inside. The value is not just “more devices.” It is a cleaner chain of evidence.

Hub, protocol, and battery reality before buying

A vacant-property sensor stack lives or dies on boring compatibility details. Before buying sensors, check four things: what hub they require, what radio protocol they use, how they are powered, and what happens when the internet or power fails.

Battery ranges vary sharply by protocol and device type. Vesternet summarizes typical Z-Wave motion sensors at about one to three years, Z-Wave door/window sensors at about two to five years, Zigbee sensors at roughly six months to two years, and Wi-Fi battery sensors as often lasting weeks to months rather than years.[9]

Cold weather deserves its own line item. Vesternet notes that cold can reduce alkaline battery capacity by about 50 percent.[9] That is not a small caveat for an unheated vacant house, garage, porch, shed, or detached building. A battery that looks comfortable on a spec sheet can become a winter maintenance problem.

Wi-Fi sensors are tempting because they appear hub-free. For cameras, Wi-Fi is often unavoidable. For small contact and motion sensors, Wi-Fi usually means more battery pressure than low-power mesh protocols. Zigbee, Z-Wave, and Thread-style setups add hub or border-router complexity, but they are usually a better fit for lots of small sensors spread through a property.

The hub decision also affects range. A contact sensor in a detached garage is not just “compatible” because the box says Zigbee or Z-Wave. It still needs a reliable path back to the hub through repeaters, powered devices, or a better hub location. Empty properties can be surprisingly hostile RF environments: closed metal garage doors, masonry, foil-backed insulation, and routers placed wherever the internet installer found a cable.

If the property already has an automation platform, choose sensors that fit it instead of building a second island of alerts. If you are still choosing the platform, a protocol-first guide such as building an interoperable smart-home security stack is the right kind of decision to make before ordering a cart full of sensors.

A workable vacant-property stack

For most vacant homes, the sensible order is not exotic.

1. Put contact sensors on the real entry paths first: exterior doors, garage-to-house doors, basement doors, and reachable windows.
2. Add PIR motion only where movement through a zone is useful and the false-alarm burden is acceptable.
3. Use mmWave presence sensors in rooms where a still human matters and where power, placement, and coverage can be controlled.
4. Treat CO2 or occupancy analytics as a commercial or managed-property option unless you are deliberately buying into that class of platform.
5. Use cameras to verify sensor events, not as the only thing standing between you and uncertainty.

The minimum useful version is an entry layer plus verification: contacts on key openings and at least one camera aimed at the path that matters, not at private areas. The stronger version adds a habitation layer: mmWave in the room where someone could stay still, or a commercial occupancy analytics system when the property and budget justify it.

More sensors also mean more batteries, more mounts, more mesh planning, more firmware, more naming discipline, and more ways to create alerts nobody trusts. A layered setup is worth building only if each layer answers a different question. Did an opening change state? Did something move inside? Is a person still present? Is there evidence worth acting on?

So the best smart sensors to monitor vacant property for squatters are not a single product category. Start with contact sensors because entry is the cheapest reliable event to catch, but do not mistake entry detection for habitation detection. Add PIR where movement alerts help. Add mmWave where static presence matters. Consider CO2 or occupancy analytics only when a commercial-style habitation signal makes sense. Then use cameras for verification. That stack does not “prevent squatters.” It gives the owner earlier, better-classified signals about entry, movement, presence, and evidence.

References

1. Aqara Door and Window Sensor — Aqara
2. Motion sensor false alarms — Reconeyez
3. Smart Presence Sensors — Homey
4. Presence Sensor FP2 — Aqara
5. SwitchBot mmWave Presence Sensor — SwitchBot
6. Squatter Detection — Alertify
7. Occupancy Sensors: Types, Accuracy, Privacy, and Picks — Occuspace
8. Dwellwell Analytics Introduces First AI-Driven Sensor Technology to Protect Real Estate Against Unauthorized Occupancy — Dwellwell, August 31, 2023
9. Battery vs Mains Powered Sensors - Pros and Cons — Vesternet