the thing i am talking about is like the facebook feature that when you log-out in one tab, the others will be "blocked" with a message "you have logged out, please login to access this page".
The current behaviour on Moodle is that it only occurs when you try to follow a link in the page or enter another url on the location bar. But if the user does not "change pages" it will have access to content even if user logged out in another tab.
I don't think that's really a security feature as much as it is a UX feature. showing an AJAX pop-up when logged out in another tab just prevents the user from seeing a nasty AJAX error, or page error when trying to interact with an invalid session.
There's nothing to stop the user from having saved in any way (ctrl+s, screen shot, etc...) the information that was already on the screen.
I agree with Danny, this new feature request for indication of session status is more UI related, I guess it would be implemented in the theme. The problem here is the performance cost - each open page would have to repeatedly query the server asking if session is active. If anybody decides to implement this please keep in mind that the session query script must not use standard sessions because it would break session timeouts - use NO_MOODLE_COOKIES, access the cookie id directly and look it up in sessions table.