Ha!

I visited the library this afternoon to return books and check out some more, as well as some audiobooks (to listen to while I drive to and from NM for Thanskgiving).  While I was in the stacks, I saw the book they said I hadn't returned -- right on the shelf where it belongs.  I took it over to the librarians, and one of them made sure it was no longer listed as "missing" in the system.

It's nice to be vindicated.