I would imagine its one of your officers posting in the hidden forum section and you don't have the correct permission to see it, hence to you it appears no new message is there but it still puts one up anyway.
I seriously doubt that the programmers put any code in to check on every post if all players had permissions to view the post and then only show the scroll to the ones that could see it, its easier for them to just put the scroll up and the ones that cannot see the hidden forum just have to get on with it.
~Baine