The metal detectors at FRA are equipped with a "random beep" feature. Some percentage of people where the detector senses no metal gets beeped anyway for a more thorough check.

I once went through a metal detector at FRA while wearing flip-flops, shorts and a t-shirt, with absolutely no metal on my person except the iron in the steak I had for lunch. Seeing my look of utter disbelief, the security guy explained that I was probably a random selectee.