I heard a little about these sorts of devices that use radio signals to see movement through building walls.

The Range-R is a hand-held radar device manufactured by L-3 Communications capable of detecting motion through solid walls. The device operates as a finely tuned motion detector, using radio waves to detect the presence of people and movements as small as human breathing at a distance of 50 feet or more.

This is similar, I think, to stories you might have heard of wifi signals being used to map human locations in a room.