Ive set up a private forum and only the user roles listed can access it via the forums page, it does not show for guests for e.g.
however if i create a post in the private forum the content is displayed within the newsfeed to all users including guests, this means that they can read the content of the forum post as it displays an excerpt in the newsfeed activity, if they click it they get an access denied message but the newsfeed needs updating to support the private forums/groups feature