The default behaviour for the new posts and things in Vbulletin up until 3.5 was to use cookies and time out the new threads even while a user was on the forum. Then once you leave the forums all threads are marked as read until new posts are made after you leave the forum.
There has been new options added and the one I have set it to should act like the old forums. The problem with this is that it's more system intensive, in fact the read posts marking on the old forum is the cause of the performance issues we had.
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