unless they come back within 15 mins (I think thats the timeout) it already happens. If they log out, it happens. its all time based on last visit. its not really unread. there is a good wiki write up on the operation

http://wiki.simplepressforum.com/doku.php?id=howto:unreadposts

if there was a button, what would you propose it do?? topics are not marked as read or unread in the db. only a last time since visit.