I still couldn’t locate the offending thing. But I finally realised through a general dbx search of my computer that the “computer.dbx” was a folder of mine, which, as the name suggests, stored mail relating to computer stuff. The folder was empty, so I lost all my mail in there. No great tragedy.

Thank you for all your help. At least I learned something about dbx’s today! I’m very curious what could have caused it in the first place, but I have no way of finding out. A weird mystery.