Bought a £10 B&Q AC Cable Detector this morning with the hope that it would detect and locate (reasonably accurately) buried mains cables near where I want to drill into the wall. It's so sensitive that it detects the cables from about 20-30cm out from the wall. Shielding the device with aluminium foil stops the detection, but uncovering even a small part of the device, and it starts beeping again. Will now try making small holes in the foil with a needle to get an optimum sensitivity.
There must be a use for this sort of sensitivity in the HiFi area! I'm sure it could detect if one wasn't using gold interconnects!