I've been opting out if I can't get through a metal detector, but my observation traveling with my mom going through the AIT is the same as the OP. I have been with her six times, and only once did she "alarm." In that case, the groper just reached out and started patting her hair and her necklace. She wasn't expecting it and wasn't warned -- I saw her startle and recoil slightly. (Some people might caption her expression as "What the heck???") When she asked what had caused the problem, the screener said "It's your hair."