It happens on the way to somewhere. The sky goes grey over the car park, or the weather app buzzes with a storm warning, and a picture forms of the bedroom window you opened this morning to air the room. You are fairly sure you closed it. You cannot remember closing it. Your phone could show you the room, if you had a camera there, and you would still be squinting at a curtain on a small screen trying to decide whether that edge is a gap or a shadow.
The standard smart home answer is a contact sensor: a magnet on the sash, a reed switch on the frame, and a report whenever the two part. For a front door it is excellent, because a flat has one front door. A flat has many windows. Covering them all means a sensor, a battery and a pairing for each, and the batteries die one at a time over the following two years.
The shape of the windows matters too. A tilt and turn window opened on tilt moves only at the top, so a magnet mounted at the handle side can stay close enough to report closed. A sliding balcony door needs a sensor where there is barely room for one. A roof window is out of reach. And the question you actually have is not whether one magnet moved. It is whether the room is open to the weather, which you would answer in a second by looking.
The gap in one line. A contact sensor answers for one sash. The thing you worry about is a room, visible in a photograph, with several ways of being open.
Stand in the doorway and look at the window. Without walking over you know whether it is open, and you are reading a handful of visual facts. A vision model reads the same facts from a still photograph.
| What you look at | Closed looks like | Open looks like |
|---|---|---|
| The sash against the frame | Flush, one continuous outline | Stepped out of line, a second outline inside the first |
| The edge | No gap | A dark or bright strip along the top or the side |
| The handle | Pointing down on most European windows | Sideways (open) or up (tilt) |
| Curtain or blind | Hanging straight | Pulled towards the opening or lifted by a draught |
| The sill and the floor | Dry | Rain on the sill or a darker patch beneath the opening |
The app takes a still photograph on a schedule you choose, sends it to a vision model with the sentence you wrote, and gives you an answer in plain words. There is no feed to watch and no training photographs of your own windows.
Scenarios are written the way you would brief a neighbour with a spare key. Describe what closed looks like in your room, and name the windows if the phone sees more than one.
Each is a separate check with its own schedule, and they can share one phone. The first one is the workhorse. The third is the one people add after the first wet carpet.
Privacy, since it is a bedroom or a living room. The phone takes a still photograph on the schedule you set. No continuous recording, no stored video, nobody watching a feed. What leaves the flat is a sentence about a window.
Checks are paid for one at a time. The starter pack is 2000 credits for 2.99 euro, the light model costs half a credit per check and the fast model one credit. New accounts get 50 free credits without a card.
| Schedule | Checks per day | Credits per day (light) | Roughly |
|---|---|---|---|
| After you leave and mid afternoon | 2 | 1 | about 50 cents a year |
| Hourly, 08:00 to 20:00 | 12 | 6 | about 30 cents a month |
| Every 30 minutes, round the clock | 48 | 24 | about 1.10 euro a month |
| Every 10 seconds | 8,640 | 4,320 | not a window schedule |
A window does not open itself between one second and the next. Two checks a day at the moments you actually worry cover the question for less than one contact sensor battery.
Can a camera tell me if I left a window open? Yes, for any window it can see: the sash out of line, a gap along the edge, the handle position and a pulled curtain all show in a photograph. Watch it from an angle so a tilted sash is visible.
Why not a contact sensor on every window? For two windows, do that. For many windows, roof windows or tilt and turn sashes, one phone that sees the room answers the real question with less hardware.
Will it tell me when it starts raining? It does not read the forecast, but it can see rain on the sill or water under the window in a daytime photograph.
Does it work at night? Only with some light in the room. A phone camera is not infrared.
How often should it check? Twenty minutes after you leave, plus one check in the afternoon. Hourly for a flat left empty for a week.
What does it cost? Two light checks a day is about fifty cents a year. There is no subscription.
Can it close the window? No. It tells you, early enough to ask someone who can.
The window check usually lives next to two others about the state of the home rather than an event in it: is the door actually locked and did I leave the stove on. If the worry is what the weather does once it gets in, water on the floor is the same check pointed downwards, and the cost per day arithmetic is worked out on its own page.
Stand a spare phone where it sees your windows from the side, write one sentence describing what closed looks like in that room, and set it to check twenty minutes after you normally leave. Most days you will not hear from it. The grey afternoon you do, you will be glad it looked.
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