How fun! Once, after climbing around in a dusty old church attic like this, I had to head straight to the airport for a flight. Barely made it on because something in the dust kept flagging as dangerous. They were using a test strip on my backpack and clothing and analyzing it in some kind of machine. Stressful as I was already pressed for time! Finally got a clean test and they let me through.
Ion mobility spectrometry is the tech that powers these, and they're VERY sensitive and VERY prone to false positives because it's just looking for ion signatures ("I smell a nitrate/glycerin/etc."). I'm not sure if they're changing modes on the machines between swabs or something, but the machine is a cue for more in-depth inspection, not "zomg I found a bomb". So, some repeated scanning to rule out a false positive or burn off gunk that's accumulated in the machine i.e. we just got a speck of something on the wand or something is gunking up the scanner as opposed to "someone covered in residue that is scary".
Also, it's not illegal to travel when covered in nitrate dust. Maybe you really were working with explosives or fertilizers earlier in the day. So it really should be just a flag to check that you don't still have the explosives with you.
Do you know if they also scan for drug residue using those devices? A while back I was travelling through a German airport, and got pulled aside to get the skin on my stomach and the inside of my bag swabbed, and I assume tested in the same kind of machine. I don't know what I did to warrant a further inspection, but I wasn't going to make a fuss. I sometimes wonder what would have happened if they'd pulled aside one of the guys in my group who were definitely using drugs
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