Must be aliens. I'm very much afraid you're going to have to take your computer out to the burn barrel in the back yard to prevent the rest of the planet from being infected...
All seriousness aside, that's a new one on me. I can't think of anything to suggest, unless you back everything up, uninstall Lightroom completely and reinstall it. Or, before you do that, maybe export some images as a catalog or create a new catalog and try importing into the new catalog and see if it persists.
Mike.