I guess no-one can work in any library without removing all sorts of 'inserts' when books are returned. I've heard of some pretty strange ones, as well as the humdrum bus or train tickets, fruit skins, and odd bits of unidentified things.
Today at the Day Job Library returned a senior math text book (2004 edition) containing - and I swear this is true - the business card of a very, very prior "ex" who I haven't seen for over 15 years.
How's that for weird!!!!