Researchers in England have created a prototype surveillance device that can be used to spy on people inside buildings and behind walls by tracking the frequency changes as Wi-Fi signals generated by wireless routers and access points bounce off people as they move around.
The device, which is about the size of a suitcase and has two antennae and a signal processing unit, works as a "passive radar system" that can "see" through walls, according to PopSci.com. It was able to successfully determine the location, speed, and direction of a person behind a one-foot-thick brick wall, but can not detect people standing or sitting still, the article said.
Source: CNet. Read full article. (link)