Well yes the ones used in security checkpoints are probably only set to detect big pieces of metal like guns and knives. It's a pain if they go off every time s

Well yes the ones used in security checkpoints are probably only set to detect big pieces of metal like guns and knives. It's a pain if they go off every time someone walks through with an earring or a metal button or zipper. But people do use metal detectors to reliably find bottle caps, jewelry, coins, etc, under several inches of sand and soil.