Metal detectors can be fine tuned. I'm used to them going off when I go through then when they wave their magic paddle of infinite wisdom around me it goes off in the places I don't have any hardware at all. Not my nips, not my ears, not my right shoulder with 7 anchors and a screw.. my shin, a foot, my hand without my wedding band on it. The TSA guy got the answer he wasn't expecting with me since I was down to my boxers and socks in the airport.. he waves over my shin, it goes off.. 6 TSA people standing around watching this fella do his gig.. my wife and about 50 people watching as well.
TSA guy: "Whats in your leg to make it go off?"
Me: "Nothing." and now I'm smiling because I can't believe my leg set it off but 12ga nip rings didn't.
TSA guy: "You have to have something, the metal detector went off."
Me: "I have only had my shoulder rebuilt....."
TSA guy: "So whats in your leg?"
Me: "Well, I did have this dream about aliens and a probe. Have you come to retrieve it? If your not my pregnant wife is looking uncomfortable with me being almost nude with another man"
I was basically told to get dressed and leave at that point..