Read this to understand how Recco works.
Basically, there's a diode that connects two aerials that make a radar harmonic in the reflector. The search unit sends out a radar beam that bounces off the reflector, and the returned waves are translated into audio signals.
However, almost all electronics have some sort of diode. The range is much less, because it isn't optimized for the same frequency, but it does work if you've got a decent idea where the Gopro/phone/ipod is located.