1
I figure that tracks
I dunno, I used to leave thin bookmarks like that in library books regularly. It's a thing, it isn't just me. Used to be all kinds of book clubs would have little paper bookmarks done up specifically to make them available without any extra effort on a borrower's part.
Hell, I went to the local library a few months ago, dug around for a copy of my book while I was there (that I wrote, not that I used to own lol) to see if they still had a decent condition copy. There was one in it from the book club I was in at the time. Which, if it hadn't seen some wear, I might have felt bad lol. But the bookmark was showing its age, and it was obvious that it had been used over the years.
I've heard some bookstores used to do it as well, as advertising. The owners would go borrow some, leave bookmarks/ads in them and return them. Librarians would leave them in place the same as the club markers because they hate dog eared pages.
Could be adhd related too, but it could be there on purpose, regardless of the neurostate of the person that left it