Don't know if anyone else has seen this, but on occasion I get a situation where if I open the site, then decide to go somewhere else, even without reading any threads, when I come back the site will be marked as read. Then the only way to know what threads have unread threads for me is to look at the date/time. At no time did I mark the threads as read. This is with Firefox, just about any version I have used. It doesn't happen every time I do this, but enough to be annoying.

 

I definitely recommend dumping all cookies, logging out, and logging back in. This sounds like a cookie issue to me...