I have a long running battle with them right now - I was travelling as pax back in March through one of their stations, when the GE Explosives machine took exception to my laptop bag. I'm trying to obtain the trace report etc as nobody was willing to tell me what the indication was & they allowed me to continue (go figure!!)
A few years ago at SFO my laptop bag got randomly checked. Swab taken, from the handles and openings, and then inserted into what looked like a dumbed-down gas chromatograph. I asked what they were looking and they said `explosives and firearms'. My bag passed clean, and off I went onto the flight.
The hilarity was that I stank of firearms residue. My laptop bag had been sat next to me at a firing range I'd visited, perfectly legally, on the way to the airport, where I had shot 3 boxes of 9x19 and 3 boxes of .38SPL. I'd been handling live and spent ammunition, and then carried the bag, so it is hard to see how the handles weren't contaminated.
Since then I've been very cynical about the efficacy of such tests.