From what I've tested, it looks like the "Messages have been posted since you loaded this page" message displays any time you are not on the last page of a thread regardless if there's actual new posts. Shouldn't this check to see if there are actual unread posts in the thread before loading? It's confusing otherwise.
Pretty sure it just checks against the last_date hidden input, which is the timestamp of the last post on the current page. So if your on page 3 of 8 (even if you've actually read all the posts), any posts from page 4 and beyond would count as new. Already noticed this on the first day, but later didn't mind it as much as I thought it's a nice reminder for those people who don't bother reading the rest of the messages before posting.