The most common field failure of a WiFi presence sensor is not a person. It is a bookshelf.
What the baseline actually is
A room has a standing radio field: the router's beacons, the reflections off walls, the metal in a cabinet. Our unit listens passively and compares each interval against the quiet shape of that field. When a person walks through it, the received level shifts. That comparison is the whole detector. Which means the quiet shape is a piece of state the device has to maintain — and anything that changes the room changes that state.
Move a bookshelf, bring in a metal filing cabinet, rotate a sofa, replace the router, or even leave a large suitcase in the wrong place, and the quiet shape is no longer quiet. A naive implementation reports continuous activity for hours. That is worse than reporting nothing, because a family would read it as "she is restless today".
What we do instead
We treat large, persistent shifts in the baseline as a recalibration event, not as a burst of movement. The distinction we use is time: a person crosses the field and it returns; furniture changes the field and it stays changed. A slow-moving offset is absorbed into the new baseline; a short disturbance is reported as activity. When the shift is too large or too abrupt to classify confidently, the board shows "no signal" rather than guessing — the same state we use for a power cut or a dropped link.
We also publish the maintenance cases plainly: replace the router or change the WiFi password and the box needs pairing again. That belongs in the manual's main text, not a footnote discovered after purchase.
Interference we do not hide
A fan, a robot vacuum, and a pet all move the field. Placement guidance exists for exactly this: keep the unit away from things that move on their own. We do not ship a "pet filter" trained on a handful of labelled homes, because a filter that occasionally suppresses a person is worse than an honest limitation. Two units in one flat do not coordinate either — each reports its own room. Merging rooms would invite conclusions about a person moving between them, which the signal does not support.
The boundary stays put
Four states: moving, still but present, no one, no signal. No state called "normal" — that would be a verdict. No score, no trend line, no "unusual day" flag. We do not measure heart rate or breathing, do not claim to detect falls, do not send alerts, and do not offer an emergency response. Not a medical device, no diagnosis — in an emergency, call 120. One box covers one room: about 10 metres in the same room, about 5 metres through one ordinary interior wall.
The unit is 499 CNY including the first month of service, then 30 CNY a month. The public board at annest.cn/live/ reports from a real occupied flat, about once a second, not from a recorded loop. Calibration is boring work, but it is the difference between a display you can trust and one that quietly lies about an ordinary Tuesday.
