The Fox and Hound. I have one of those (not that brand). It was invaluable when I was doing the infrastructure wiring (TV, phone, security) in my house.Thanks Ron. Basically, it appears to be similar to the 'Receiver Wand' in a cable tracing 'Toner' set.
http://www.amazon.com/Cable-Tracer-Tester-Generator-Signal/dp/B001MB759U