Was it an airport without a dedicated prescreen line? I’ve personally never had this happen at any of the dozen or so airports I’ve gone though, but they were all large airports.
Unless things have changed in the past several years, you should be able to opt out of the mm-wave scanner for a pat down instead. Although I guess you could do this without pre-check/GE. It just takes more time.
Between the two, I have managed to never walk through a mm-wave scanner.
As an aside, why are you in precheck if you have GE? GE gives you everything precheck has
I have had to go through mm-wave scanners in MIA and MCO. I am sure I could have opted out if I were willing to go through the process of explaining things to the TSA employee instead of just rushing through.
I am in both because I enrolled in TSA Pre and then about a year later needed to be in Global Entry due to a sharp increase in international travel. Once my TSA Pre expires, I don't expect to have both. Right now, various frequent flyer programs still have my TSA Pre number entered and I just leave it alone.
Unless things have changed in the past several years, you should be able to opt out of the mm-wave scanner for a pat down instead. Although I guess you could do this without pre-check/GE. It just takes more time.
Between the two, I have managed to never walk through a mm-wave scanner.
As an aside, why are you in precheck if you have GE? GE gives you everything precheck has