Hello !
Not sure that i understand this question correctly - sorry

But to marked as read thread need:
- user opened showthread page after last post in that thread
- or last post in forum was in that thread when user desided to "mark read" that forum or all forums
- or last post in thread older then number of hours specified by user in his CP options and if that number of hours is not zero
under any other condition thread will be marked as unread