Phantom metal

Today I was at the security lane at CLT (not important but including it all) I have precheck, which I somehow wasn’t aware of so I had everything out and I was going to the body scanner, when I was redirected to the metal detector. I walk thru the metal detector and it makes the noise that it makes when it detects metal even though I had nothing in my pockets but I wasn’t randomly searched. I was just wondering, what might have set this off? I had no metal on me or in me so what would have set it off (the next time I went through it nothing happened)