Actually it would show unread since last visit, or rather since you were last active.
Last night I made sure to log-out of the forum (top right of the page)
the computer was off during the night
When I logged-in this morning
ALL threads had the "Contains no unread posts" icon
and yet there were new threads and old threads with new post made during the night when
looking at the date and time of 'last post by' - right hand side
If you stay logged in and leave the window/tab with the forums open long enough that last active date and time will get updated in the database, as there seem to be a periodic check on "who's logged in now" (or something like that), and not only updated on actual page loads.
Only after a 'new' thread is started or a 'new' post is made to a 'old' thread from the time I logged-in
will that thread show the "Contains unread posts" icon
Over time some but not all of those icons will change to the "Contains no unread posts" icon
as I hit the reload key about ever hour
For the last few days I have been looking for a way to tell which thread/post I have read
Thanks