Someone Plus Black Marker Equals Censoring

A library in Tennessee has a problem with censorship: someone is taking black marker and crossing out bad words.

Well, at least they're enterprising - at least, if it's just one person. One book at a time, they eliminate questionable words, like some kind of really crappy superhero. Thank you for protecting the rest of us from foul language!

How would you catch the person doing that? Are the books being checked out? If so, can an account be flagged as suspicious?