This is pretty cool

[mapes]

Using wifi to track movement through walls

Wireless Network Signals Produce See-Through Walls | Threat Level | Wired.com

Researchers at the University of Utah have found a way to see through walls to detect movement inside a building.

The surveillance technique is called variance-based radio tomographic imaging and works by visualizing variations in radio waves as they travel to nodes in a wireless network. A person moving inside a building will cause the waves to vary in that location, the researchers found, allowing an observer to map their position.

[Trooper110]

I read about that....it's cool but they could only narrow the range down to a meter from what I can recall. Seems to me that for most purposes you'd be using it for that that would be a little wide, unless you only want to count moving bodies.