Don't use your luggage to carry Christmas firecrackers. The residues can stay there for months or years...

I guess that I should probably not travel with the briefcase that I used when I was in the oil field as a perforator. It was used to carry shaped Charges, or primer cord, or blasting caps. (but not all at once.) It was also carried on jobs where we were exploring for Uranium. Guess if I want my annual prostate exam done by a minimum wage felon, I could carry it.
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I had a positive reading once, one guy swabbed it , handed it over to the other, who developed it and it was positive. They were training the new guy and wanted to see what he would do. It was explained to me and I was let through as soon as he recognized it as positive and asked for the secondary search. And, no, they did not swab it again, but they don't do it every time.

I had a false positive for TNT a month or so ago in CLE. The guy shouted some code frantically, and another security droid came over, along with a supervisor who tried to explain to me what was going on in an extremely calm manner, as if to prevent me from using the triggering device. :-)

Of course, they retested things and sent me on my way, but it was a humorous false positive at the time -- everyone was very nervous as they recalibrated the machine.