[Dovecot] phantom new message notification

I've developed a phantom new message notification on a single IMAP folder in Thunderbird. I've deleted the Dovecot index files for this problematic folder, and I've deleted the .msf file in the Tbird profile. Neither of which solved the problem. The only other time I've run into something similar was with CONDSTORE quite a long time ago, but that affected all folders, not just one.

Any ideas?

-- Stan