This might explain things for a lot of users who wonder how the forum determines what they have read, and when to mark what they have seen as read posts. A few people are still having some issues and they are being investigated. This is what should be happening though:

What Invision Board does is checks your last login time in the forum
and lists all threads which have been updated after that. It doesn't
check wether you have read them or not. This login time is updated in
2 conditions either you logged out and login again or you visit again after
5 mins (the default session time out)

How does it check if you have read them:
1) While you are browsing it will setup a temporary cookie which holds
the thread ids with the time which you have last seen.
2) When you close your browser this cookie will be deleted.

Now invision board expects you to come back after atleast the minimum
session expiry time of 5 mins or 3600 seconds. If you revisit before
that it will show all read posts as new.