[G.R.]

Jim is right phone guys and installers of other type low volt equipment and lines use these gizmos. They work off of usually a 9v battery and send a "tone" along the wire with the "reader" you can follow a wire to it's source or in some cases a break in the wire. The one phone and wiring pros use are rather expensive but for about $30 you can get one that works well from places like Radio Shack. I bought one about 10-12 years ago and have used it to trace hidden wiring in boats and recently on the car I bought to straighten out the wiring.
Easiest way I've found to trace wires