That is an interesting scenario. I am afraid there is no stealth function, so if you add yourself to a public Topic, everyone watching the Topic will see that you are watching it too. Topics can fucntion as group workspaces, so the team always knows who is in the Topic/available/active/passive.
However, the watchers of a Topic will not get notified that you have started watching the Topic, if that was your concern.
Hope this helps :)