Rants of a future librarian

You know what annoyed me today? When I found that someone had put checkmarks next to certain characters' names in my copy of "And Then There Were None". Was the previous reader checking off who dies or marking possible people who might've "dunnit"? I'm guessing the first. Luckily I only caught one name before I stopped looking...but seriously...use a list instead of marking a library book! *sigh*