Comment by xkcd-sucks

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