Unbeknownst to Ristoro, there was a small hole in the wall near the bookshelf where she was working. Inside that hole, Professor Nibbles was muttering to himself, as he scribbled equations on a scrap of notebook paper with the stub of a pencil.
"Hmm...If I carry the three, and then divide the result by 7.5, and convert the result to degrees Centigrade...Ah!" He dropped the stub of pencil, then scurried over to the hole (or, as he thought of it, "door"). "I'll need to get my paws on a book on the biology of crustaceans, to confirm what I have deduced about the body temperature of lobsters..."
With that, he peeked out of the hole, at the shelf where the biology books were. He muttered an oath under his breath when he saw the library worker (he had heard the head librarian call her "Risotto" or some such name) working at the very same shelf.
"Dash it all," he griped. "She has to be working there now, of all times?"
He paused for a moment, running some numbers in is head, and then said, "Bah, I'll risk it. It has been a while since I had my daily constitutional." He then slipped silently out of his hole, and started working his way towards the bookshelf, using the furniture as cover to avoid beeing seen by the library worker. The last thing he needed was for her to scream and blow his cover. After all, exterminators were so bothersome, even if they couldn't do much.