Library Ghosts

I work at a library, and when someone leaves with items that don't have cards in them, a loud beeping sound is heard when they reach the exit.  Sometimes the sensor goes off when no one is anywhere near the door; is this a random occurrence or proof that the library has ghosts...and they're stealing books?  ; )

If you've been to a library that actually is haunted, leave me a comment...I'd love to hear about it!