[amykhar]

From what I'm seeing, it looks like it's in a cookie.

in showthread.php, you'll see,

$threadview = intval(fetch_bbarray_cookie('thread_lastview', $thread['threadid']));

and

if ($displayed_dateline <= $threadview)
			{
				$updatethreadcookie = true;
			}

I found this by going to the forum settings, seeing that jelsoft called it thread marking and searching showthread for marking and then following the code.