Extra guests, parties and why you must never hide a sensor

Two related worries come up constantly among hosts. How do I know whether more people are staying than booked? And how do I know whether a gathering is being held in my property?

Both are legitimate concerns with real financial consequences. Both also lead directly to the single most damaging mistake a host can make, which is concealing a monitoring device. This article deals with what works, and states clearly where the line is.

The hidden device question, answered directly

Concealing a sensor — behind a picture, inside a smoke detector housing, in a clock, in a speaker — is the wrong answer, and it is worth being unambiguous about why.

The practical point is that concealment gains you nothing. A disclosed, visible sensor works exactly as well as a hidden one for every legitimate purpose. The only thing concealment adds is the ability to monitor someone who has not agreed to it, which is precisely the thing that is prohibited.

What you can legitimately detect, and how well

Carbon dioxide, which is the honest answer

CO2 rises with the number of people breathing in an enclosed space, which makes it a genuine occupancy indicator rather than a proxy for something else. A property booked for two that shows CO2 levels consistent with eight people is telling you something real.

Its virtues are considerable: it identifies nobody, records nothing about behaviour, is entirely uncontroversial to disclose, and doubles as useful air quality information. Its limits are equally real — ventilation, room volume and open windows all affect the reading, so it indicates rather than proves.

Presence sensors

Motion and mmWave presence sensors tell you a room is occupied. They cannot count people reliably, despite marketing claims to the contrary in some products. Useful for knowing which spaces are in use, not for establishing guest numbers.

Noise level

Covered in noise monitoring and the law. Sustained elevated levels late at night correlate with gatherings, and correlate is the correct word.

Network device counts

Counting devices connected to the property Wi-Fi is sometimes suggested. Be careful: it is unreliable, since one person may bring three devices, and it edges toward monitoring guests' personal equipment. We would not build on it.

Door activity

Frequent entries and exits over a short period, particularly late at night, is a reasonable corroborating signal. On its own, unremarkable.

Water and electricity consumption

Genuinely useful and entirely uncontroversial. Eight people use markedly more hot water than two, and consumption data is something you already have if you monitor energy. It is also retrospective, which limits its use during a stay.

Corroboration is the whole technique

No single measurement establishes that a property is over-occupied or that a party is happening. Two or three agreeing is a much stronger position.

A gathering typically produces: CO2 well above what the booking would explain, sound levels elevated for a sustained period late at night, repeated door activity, and elevated water and electricity use. Any one of those has an innocent explanation. All four together, on a Saturday night, does not.

This matters practically as well as evidentially, because a response based on one signal is frequently a response to a guest who cooked a large meal with the windows shut.

Responding proportionately

Start with a message, and assume innocence. “I noticed the property seems busy this evening — just a reminder the booking is for four guests and the house rules ask for no additional visitors.” A large share of cases resolve there, and framing it as a reminder rather than an accusation is what makes that work.

Escalate to a call, then to attendance. A local contact who can visit is worth more than any sensor at this point.

Involve the platform when house rules are being clearly broken, not on suspicion. Platforms generally handle unauthorised-guest and party violations, and their process works better when you can show what you observed.

Do not attempt remote enforcement. Turning off the heating, cutting the hot water, changing the door code mid-stay or disabling the Wi-Fi is a serious matter, potentially unlawful, and dangerous if it affects heating or exit. It is also a guaranteed platform violation. Whatever the provocation, do not.

What about outdoor spaces?

Gardens, terraces and balconies deserve separate thought, because they are where most neighbour disturbance actually originates and because the privacy position differs.

An outdoor decibel sensor is generally less contentious than an indoor one, since a garden is overlooked and the expectation of privacy is lower — but it still requires disclosure, and it still must not record audio. Outdoor sound also carries much further at night, so a level that seems modest on the terrace may be exactly what the neighbour is complaining about.

Cameras covering outdoor space are a different matter and need real care. A camera at the entrance, disclosed and aimed at your own doorway, is generally acceptable and is standard practice. A camera overlooking a garden that guests are using as private outdoor space is not, and one that captures a neighbour’s property brings data protection obligations of its own. Aim narrowly, use privacy masking, disclose it, and never point anything at a space guests would reasonably treat as private.

Prevention, which works better than detection

Hosts who have few problems with over-occupancy and parties generally prevent rather than detect.

State maximum occupancy and no-visitors rules explicitly in the listing, before booking.

Use minimum-stay requirements. Party bookings are overwhelmingly one and two nights at weekends. A three-night minimum eliminates most of the risk in properties where this is a problem.

Price to discourage the wrong booking. Per-guest pricing removes the incentive to under-declare.

Communicate before arrival. A host who has exchanged messages is a person rather than a listing, and that changes behaviour measurably.

Tell neighbours how to reach you. They will know about a gathering before any sensor does, and their willingness to call you rather than the council is the outcome you actually want.

A setup that is both effective and defensible

1. House rules stating maximum occupancy and no additional visitors, in the listing.
2. A minimum-stay policy if parties are a risk in your location.
3. A CO2 sensor in the main living space, visible and disclosed, doubling as air quality monitoring.
4. A decibel-only noise sensor, visible and disclosed, in the same room.
5. Energy and water monitoring, which needs no disclosure debate at all because it measures the property.
6. Nothing hidden, nothing in bedrooms or bathrooms, no audio, no cameras indoors.
7. A local contact who can attend, and neighbours who have your number.

That gives you corroborating evidence, a proportionate response path, and a listing that survives scrutiny. Every part of it is something you can describe openly to a guest before they book — which is the test worth applying to any monitoring decision in someone else's temporary home.