Last I checked, most cameras can't see through walls. "Something in the way? No problem. A pair of MIT researchers wrote in 2013 that they could use a router t

Last I checked, most cameras can't see through walls. "Something in the way? No problem. A pair of MIT researchers wrote in 2013 that they could use a router to detect the number of humans in a room and identify some basic arm gestures, even through a wall. They could tell how many people were in a room from behind a solid wooden door, a 6-inch hollow wall supported by steel beams, or an 8-inch concrete wall—and detect messages drawn in the air from a distance of five meters (but still in another room) with 100 percent accuracy. (Using more precise sensors, the same MIT researchers went on to develop systems that can distinguish between different people standing behind walls, and remotely monitor breathing and heart rates with 99 percent accuracy."