I was rather intrigued by this recent, highly publicised, paper:
Through-the-Wall Sensing of Personnel Using Passive Bistatic WiFi Radar at Standoff Distances
[url=http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6020778]Error Page - IEEE Xplore
which claimed to detect presence using Doppler analysis of ambient WiFi signals at 2.4 or 5 GHz.
Not that there’s a Z-Wave module for that yet, but I’d quite like to prototype one…