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