Some weeks back I read a post here about detecting people in rooms by measuring how the physical body interferes with the wifi signals. I wouldn't have imagined someone could extract useful information at this small of a scale. wow!
http://www.theverge.com/2015/10/28/9625636/rf-capture-mit-wi...
It can detect people and track their movements behind walls, and tell different people apart.
It can also measure breathing patterns and heart rate.