Some may call this a feature, but for me it is a bug.
I have a hidden forum, where moderators can discuss things about the forum and the users.
When a 'regular' user checks the active users page, he can see which users are currently watching each topic.
From this screen, they can also see the subjects of the topics in the hidden forums (and hidden topics as well I imagine), that are currently being watched by a moderator.
So, if the topic subject is 'revealing' enough, they can make good assumptions as to what is being discussed behind their backs.