Because the forum is hidden think of it as an invisible forum, one that doesn't exist. If the forums show up in the drop down search or move option users who do not have access to the hidden forums will know they exist.
I think however that you can go into your CP and move the topics under Maintanence. In there you should be able to see every forum created. I think you can also take out the Search and drop down codes from the Hidden Forum hack so that your users can see the forums.
