Can't a thermal camera can do this? It should be able to detect any difference in ambient temp, even a few degrees, and a CMOS sensor gets warm, not to mention any wireless transmitters, power sources, and ICs. Turn off everything in the room and walk around with a ~$100 ali express handheld thermal camera.
Ideally yes but in practice thermal cameras measure radiance not temperature and different materials are more or less radiant; putting a camera behind a wall blocks the signal or at least diffuses it; the resolution+SnR are not great at consumer price point; and background objects can be surprisingly warm. For example a tree at night can be about the same temperature as a wireless camera attached to that tree. So in the best case it can work to see power-inefficient devices out in the open, but isn't useful under many real world conditions