Man, I'm having the same thing! It comes and goes for me. Usually, it's the Dog in Need, but others like the Support forum says there are new messages (when I've viewed them all), also.
I'm wondering if it has something to do with people having modified/edited their posts? So, they're not new, but since something changed, the "clock" says that there's a new file? (The server "clock" is how a lot of FTP clients determine new/old/modified status of files, so just guessin'.)