This odd message appeared today regarding one visitor when I was looking to see where they were at. What part of the forum is invisible to the administrator?
It is not invisible. When a user does an undefined action, or encounters an error, SMF can't log his action properly (since there is no proper action at all!). Thus it shows "nothing". As you can see, it is
"nothing, or nothing you can see". That means, it is not always something you can't see. It is sometimes really "nothing" :)
It must have been an error, then. Even if a person just logs on and stays looking at the top-level forum directory (doing "nothing") it will show. So far, I have not encountered any errors at all, myself.