Im one of those mysterious hidden users. Why? Because i just randomly chose that option when i first registered in. Thought behind it was if i remember right, that usually when i log in, i go do something else while at it and thus i can just be idle for hours here as logged while not actually surfing forums.
Also in this hidden form i am naturally invisible and i can walk behind you while you write messages into these forums. So dont be suprised if you get this feeling someone is behind your back, watching, waiting, observing.