Well you know, at the library we already have ways of making sure people bring things back. Most of the materials patrons borrow have been coming back for centuries now. It's how we work.

The way to keep the locks from disappearing is to either check them out on the library card, (like when checking-out a book or video) or have the patron exchange their card for the lock, (like when they're using reference materials).

If they don't come back, it's procedurally the same as any other lost or overdue item. We put on our Library Raid Jackets and bust down some doors!

I work part-time at my neighborhood branch of the public library. I've already sent the suggestion to my boss.