I flew out of SFO last week and set off the explosive detector after opting out. My experience was rather different. The TSA agents took my backpack and made a cursory hand-search, doing explosive swab tests on my electronics. After failing to find a source, they took me to the booth to do a more through check. They didn't ask me any questions at all about myself or why I might have set the system off other than the normal 'any sensitive areas' questions. I did volunteer that I'd spilled a small amount of gasoline on myself when fueling my car the previous day and that I was wearing new clothes that had not been washed since purchase. They seemed entirely disinterested. The whole thing took maybe 15 minutes longer than a regular opt-out pat down.

So, apparently that's how it goes when a white male sets the thing off.