I’ve heard (but not verified thru personal experience) that summer sausage (like hard salami), common in Wisconsin, sets off the explosive detectors. I often traveled with cheese curds or summer sausage; never had a problem, but that was before 9/11, when cheese was only cheese and not a terrorist weapon.
RE OP…If reality TV can be relied upon, those swabs and swabbing are for drug detection, and the machines they put them in can analyze multiple narcotics (heroin, pot, meth, anything not in the Republican playbook allowances).
I have an awesome, rugged, RAF rucksack that I bought at a flea market in Iceland. It tends to set off those machines, even after repeated washings. I don’t want to give it up though, so I just deal with the extra scrutiny.
Years ago my CPAP was pulled for a random swipe test and came up positive. The TSA agent asked me a few questions about where it had been and what it might have been in contact with (I can’t remember specifics after all this time) and then let me repack it and go on my way.
My brother got stopped when his bag set off the scanner. Of course he was returning from a 9 month deployment in Afghanistan and in uniform. After a quick look through his bag they let him through. He did ask them WTF they were expecting to happen when they tested it.