Last night at the airport, I removed all the metal from my pockets and walked through the metal detector. It went off.
I discovered the iPhone earbuds remaining in my pocket, and removed them, and tried again. It went off again.
I emptied my pockets to show the security person that I had nothing metal in them. "All I've got is paper and this bag of cough drops," I said.
"It's the cough drops," he said.
I didn't see how that could be possible, but I took the cough drops out of my pocket and tried again, and the metal detector didn't go off.
I just now looked more carefully at the bag, and it turns out that the bag's material includes a very thin (almost invisible) layer of metal foil sandwiched between the outer paper layer and the inner plasticized-paper layer. So that must be what set off the detector.
Anyway, travel tip: don't try to carry cough-drops bags through airport metal detectors.
(The brand in question is Ricola; I'm guessing other brands use similar bag materials, but I don't know.)