I was just thinking about hand swabbing. I've had this as a "random" irritation at Dulles and SFO.

Wouldn't residue be much more likely to show up on bags or belongings? Given the porous nature of hands, sweat, etc., residue would be easier to find on the surfaces of a bag, or perhaps even a coat (not an inner layer of clothing, which is what is tested in an "enhanced" pat-down.)

So the question is: why does this make any conceptual sense?

"We're being unpredictable" isn't the answer. I'd like to know a scientific rationale.