I had something similar happen with the NoS. Last month I got 'randomly' selected to get scanned after going through the WTMD in PHX. Being used to WTMD only at home and Pre everywhere else I didn't think about taking my wallet or fitbit zip out of my pocket so I had to get rescanned*. Both times it "hit" on my shirt in two places. The TSO realized he didn't have any gloves so I just pulled down the collar of my shirt to show him there wasn't anything under it. He said "good to go" and waved me through. Honestly he was as confused as I was as to what it was hitting on. I mentioned this in another thread and someone suggested it was probably sweat which makes sense because it hit where the straps of my backpack would have been.
Did the TSO let you see the screen with "Gumby" figure showing where it "hit?" If it kept hitting on the same place or if it was different each time may help explain what happened.
*I was surprised when he said to just hold my wallet and had me put just the fitbit in the dog bowl to get x-rayed. A TSO with common sense seems to be a rare breed these days.