Wifi motion detection is a thing, but I doubt it's limited to just AP. Even as that marketing picture suggests I would guess it's any wifi transmitter, clients too.
But in any case, the processing must still be done. Perhaps someone will make a detector on an esp32.
The router I suspect is used because it can more easily connect to servers to do things like this:
https://vpnoverview.com/news/wifi-routers-used-to-produce-3d-images-of-humans/
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We were doing this in the 90's with various rf frequencies.
But yes the host processing was the key.
White noise emf generators with a random freq sweeps and power outputs were useful in limitimg the capture of good info.
Also a good deterent for audio bugs....
