They deliberately scan the books returned overnight without knowing who borrowed the books or whether the books had been borrowed.
Then they wander about quite, quietly re-stacking the shelves, finding where the books go, and singing in their mind in a lovely contralto or contra tenor:
I can't speak for the rest but I start the day with a couple of shots of O.P. rum to steady my nerves before dealing with the 'I borrowed a blue book here last year, where is it?' questions.