>>509190156The wifi radio signals bounce around a bit indoors and because the timestamping of individual packets is very fine=groined it's not too difficult to detect disturbances in the timing induced by humans moving around and interfering with the radio signal.
It wasn't a conspiracy theory at all, a security researcher in (I think) Switzerland published a paper a few years ago showing how you could map out interior spaces and detect movement of up to 4 or 5 people with moderate accuracy. Cops will absolutely force this into Windows and other corporate IT infrastructure and demand use of it to snoop inside homes within the next couple of years.