"I don't really work on Sunday. Not official work, anyway." He smiled a little. "I leave whenever Dominic is ready to head out." Which could be minutes or hours away, one could never tell with Dom. "You're welcome to stay in the library, then. Someone's always around, but I will have to lock up the office."
He was quite possibly a little bit too trusting, but he was fairly certain nothing bad would happen to his books on Agency grounds, and there was a rather elaborate and effective spell on his library preventing books from leaving the building. "So, you're welcome to stay as long as you like."