When reading in a forum, after I read a thread with a "new" message, when does it become read and the little piece of paper turn white?

I've tried exiting threads by clicking on the forum name, (Eg. All Forums >> [Technical/Forum Help] >> Technical Help but when I am returned to the forum index, I still see the same posts as unread. Only when I leave and come back a couple hours later does it seem to register that i've read the post.

Thanks!