Here's something creepy: researchers at the University of Utah have developed a way to use wireless signals to detect movement through solid walls and doors.
The technique, called variance-based radio tomographic imaging, processes signals from a 34-node IEEE 802.15.4 wireless network. It's the protocol for personal area networks used by home automation systems such as ZigBee.
The basic idea is straightforward. The signal strength at any point in a network is the sum of all the paths the radio waves can take to get to the receiver. Any change in the volume of space through which the signals pass, for example caused by the movement of a person, makes the signal strength vary. So by "interrogating" this volume of space with many signals, picked up by multiple receivers, it is possible to build up a picture of the movement within it.
They were able to detect movement in a room to within a meter or so, which is pretty good. They won't be able to see what you look like in the shower, however, so I'm going to call this a good advancement. But be careful, researchers. Don't try anything sketchy. [Technology Review via Slashdot]