Modern metal detectors can be programmed to ignore common alloys in coins, etc, to cut down on nuisance alarms. The more advanced ones are also 'zoned' which means that several independent detectors are used to scan the head area, shoulders, chest, torso and so on. This means that a lot of small, randomly-distributed items of metal don't add up to a false "large item" signal.
Incidentally, Glock might not build ceramic guns but it's interesting that there's at least one airport metal detector company which highlights the fact that it can detect the Glock 17 - the implication being that this weapon is more difficult to pinpoint. I don't know whether that's a fact or whether it's using the false rumours about the Glock as a marketing tool.