By the way, I knew that Simple Machines forum's can restrict sub-forum read access to unauthorized users, but I never knew they could hide a sub-forum's existence to unauthorized users.
You know the URLs for each sub-forum end with "board,2.0.html" or something, each sub-forum with its own ID? Well, after stepping through some of them, there seemed to be some suspicious gaps where it said where it could not find or I did not have access to the board given (it did not specify which). I'm guessing your "top-secret" sub-forums are in there somewhere?