"It's... not that serious, I just need to get it when she comes out." He starts to raise an arm, as if to scratch an itch near his collarbone, catches himself, and lowers his hand. "It's for the library, you see. They put me in charge of recovering the alchemy books."

It makes sense - Percy is an alchemy teacher, and spends more time in the library than most. He probably knows that section well. And the librarians are often harsh with those who fail.