The first time I saw the swab test was in PHX a year or two ago, and the security person said that it was a test for explosives (my readout said 0/500 or something like that), which I accepted and thought not much more about; but at BWI-UA gates, where one particular bag has been tested almost every time, the security person said it was for drugs. I've never been stopped at BWI-any other gates with this bag, which is kind of odd.
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Cheers
Michael *G