Tags created on threads in private boards (or moderator-only boards) show up on the Search Tags page.
Not sure whether this should be considered a bug, but I would think thread visibility settings should also apply to the tags associated with those threads.
Use case example (fictitious, but...
What do you think about allowing members to "manage tags" on the threads that others created? May actually help to make the tagging feature useful, since some users are bound to ignore tagging on their own threads when tags would make the discoverability of those threads useful.