maurycy did it again. "your tv can see through walls" is a short one, but it stuck with me all afternoon.
he points an SDR (a cheap software radio) at a local ATSC tv station, then nudges the antenna a few centimeters until a strong reflection almost cancels the direct signal. that's selective fading. in that state the signal gets crazy sensitive, so getting up from his chair shows up on the waterfall, and walking around in the next room leaves stripes. each stripe is one wavelength of path change. so he counts pixels, does a bit of math and gets his walking speed: 1.3 m/s, about 4.6 km/h. no camera, no sensor, only the tv signal that was already in the air.
and it's not just tv. wifi and lte work too, just messier. "happy paranoia!" is the perfect sign-off.