I suppose you could install a magnet and use some sort of magnet field sensor to detect it. Alternatively - I know it's a wild thought - why not bury something metal under the floor and just use a real metal detector?
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This is because the metal detector is actually a mock up of a device that the British Army use to detect IEDs in Afghanistan. It's being built by our props department and will be part of an interactive installation at the Army Museum. We can't have it going off every time it comes it detects metal, it needs to simulate the experience of a bomb disposal squad.