FWIW, I've never seen anything fail the swab test.
My son has failed the swab test! Several times.
One year it was a previous use of firecrackers. Another time it was right after going to the range. (Boy howdy, did he fail that one... and his bags failed... and his hair and his shoes... They swabbed everything and everything came up hot. Thank goodness the people searching him had a sense of humor.)
My wife’s laptop bag failed the swab test. After prolonged questioning, we concluded it had been sitting near the gun safe, hence the result.
She was pissed when I later observed that my bag had passed the test - despite being my range & ammo-storage bag.