Majdi, I don't think you understand.
When you go into Forums > What's New
There should be a link "All Recent Messages". That link shows if there are posts that you haven't read. But if there are no posts you haven't read then the link isn't there. It shouldn't go away.
You can drill down under the Members tab to recent activity, but I don't think most members would think intuitively to look for recent activity under the members tab. Further, that shows all activity not just posts.