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The phone may have a magnetometer (magnetic compass).Putting a magnetic compass close to iron would make it go wonky. Does the phone detect aluminium ?, (try non-magnetic metal kitchenalia or aluminium drink can)If it detects non-magnetic metal, like aluminium, then the strength of radio signals transmitted-recieved by the phone may be being used to "detect" metal.
NeilI suspect your phone has a small magnetic coil of some sort that acts as the compass. Most metal detectors use a coil to generate a magnetic field, then measure the changes in that field created by nearby metallic objects. I have a VLF (Very Low Frequency) metal detector that is good up to about 10' (that's a bit more than 3 meters for the metrically non-impaired) in non-metallic soils and is pretty good at discriminating different types of metals. I also own a VLF detector, which is a geeky gadget that uses the VLF radio waves generated by the US Navy to help them detect submarines. It does a great job of finding buried locomotives and school buses, as well as buried pipes and sometimes faults- but not terribly useful for mineral exploration.Try your phone under some high voltage lines (but don't blame me if your new phone spouts flames) 
Is it possible that metal in the vicinity of the phone will absorb energy from the phones antennae, resulting in a change in current in its output stage?If so, then this could be translated into a reading on the phone's display.