For the past week or so, I noticed the normally busy email stream (mostly mailing lists) in my Google Mail account had dried up to a trickle. I didn't think much of it at the time, since I didn't seem to be missing anything important and most of what I wasn't seeing was from the various Yahoo! Groups I'm in.
A few days ago, I had some time to dig into the matter. The Yahoo! groups I'm in were still pretty active and changing one of them from digest mode to individual email mode allowed a small trickle of mail through.
GMail's Spam folder contained the usual spam email.
When I looked in the Junk label though, I found all my missing mail. Several days worth (close to 300 messages) all sitting there marked as Junk.
For some reason, something started quietly attaching the Junk label to almost all of my email. Since none of my existing GMail filters are set to classify anything as Junk, and I hadn't changed any of them recently, the prime suspect would be Thunderbird as the only other thing that touches my GMail account. I don't recall setting up any junking filters in Thunderbird, so I'll have to do some digging.