Sometimes, when there is an update to a thread that you have looked at previously, if you go and read the new post and then leave, it will sometimes a minute or two later give the same indicator that there is another new message when there isn’t.
I’m not a hundred percent certain but i think the indicator of a new message still shows when a new post in a thread has been removed by the Forum Moderators. So it looks like there is a new message but when you view the thread there isn’t one because it has been removed.
do those get removed completly?
In some cases i think so, yes (as far as i understand it at least). I’m not a Forum Mod so can’t say for certain :P
that might be the cause then