My wife used to handle library returns. They had to examine each book for signs of bed bugs, and found interesting things used as bookmarks. Sometimes money, personal notes, or random business cards.
But I tell you, no story beat: A razor blade. Yep. Just fell right out from the pages. Naked and sharp. How's that for chaotic evil??
(Thankfully nobody was harmed. Glad they all wore gloves!)
Right?! And who would have a razor so close by that it would even make a convenient "Oh I'll just use this then" bookmark? That was a weird one. I hope it wasn't some kind of cruel joke.
On the other hand, she found like $50 once. Who knew books would be risk/reward mystery chests?
In any case, once a book is returned, it's purged from the user's history for privacy reasons, so... (Shrug!)
I leave the whatever it is in the book at the end. Not in library books, mind you. Don't like making more work for librarians. I started the habit when I found a baggage claim ticket in a book I bought at a used bookstore. I leave them in when I pass them on to the next owner.
I've found a few ersatz bookmarks over the years. The best so far was a 6 of clubs. The worst was a used q-tip.