Welcome to the world of magic. (Well, RF...but it's the same thing.) These devices can sense a wire without even touching it! Used in two pieces, the transmitter connects to the wire or cable (phone, speaker, coax, etc.) and the deedle-deedle-deedle (sign-song voice) receiver makes that sound whenever it nears the wire. When it touches the wire it gets much louder. Use this (and a bit of technique) to locate and identify wires, even when buried in sheet-rock!
Bonus tip: Connect the transmitter to your speaker wires to test/find the associated speaker. It's powerful enough to be plainly heard coming from the speaker on the other end of the wire!