Harry Potter and the Half-Blood Prince
"Tell me, how have you been communicating with Rosmerta? I thought we had all methods of communication in and out of the school monitored."
"Enchanted coins," said Malfoy, as though he was compelled to keep talking, though his wand hand was shaking badly.
"Isn't that the secret method of communication the group that called themselves Dumbledore's Army used last year?"(27.187-189)
Harry wondered where Dumbledore had learned Mermish. There was so much he had never asked him, so much he should have said…. (30.123)