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I was once pulled aside for secondary screening by the TSA, and I was so bewildered that they asked if I'd taken a cab to the airport—and I had. The agent explained that it's a common cause of false positives like mine—"residue" gets left behind on the seats. I didn't feel like pressing them for more details, though.

I'm not sure what the TSA use but I'm pretty sure they don't really work.

I've had multiple friends confirm a negative test for GSR when they'd just been at the shooting range in the same jacket/etc.

We still have people accidentally bring guns/knives on planes...

Hand creams can set off that alarm.

They often contain glicerine, which can be a compund in explosives.