They don't know, they don't care, they don't do anything. Their sole purpose is to make people more comfortable with invasive government overreach.
Next morning at the airport was my first time seeing these machines. As the machines doors opened and closed, jets of air puffing at the passengers ahead of me, I wondered about the shorts which I still had on. I looked down at the empty pockets and they were literally sparkling with gunpowder and other residue from the fireworks the night before. 'Let's see what happens' was my only thought and I felt a bit let down when I got through with no problem.
Edit: Shorts sparkling in the light with some kind of crystalline residue from the fireworks, not sparkling like a lit sparkler, which would have been even more impressive.
Amen.
(Don't get me wrong: drug searches at TSA checkpoints are extremely alarming, because TSA has been given a near all-access pass to mechanically searching people).
Fun fact: if you do any welding before you get on a flight, there's a good chance you'll set those things off.