In this video Chirag Nagpal demonstrates his Sat-light project, he writes:
It uses a Beaglebone Black connected to the Internet, and download TLE values of the ISS and other satellites. It then calculate the exact location of the satellites using the Python Library PyEphem. Incase one of the Satellites is hovering above, it lights up! This is how I can keep track of when I am being observed from outer space!
Project info at Chirag Nagpal’s project page.
The source code is available on Github.