I assume that the icon in front of the topic title is green when there are unread posts, and grey if there are no unread posts. If so, it's not behaving properly at all times. For example, when I replied to a post and go back to the topic overview, the icon is still green (see screenshot below). Even when refreshing the overview by pressing F5 doesn't turn the icon to grey.
To turn the icon grey, I have to open the particular topic (and see I'm indeed the latest one that replied), go back and press F5 again. Now, the icon is grey.