Oh no, seems like this would be a problem if staff thought creating an unlisted post in general category would be invisible, but is not! Checked and same thing happened when I tried this too.
Can understand why no one noticed this before, after all there is a separate locked category that can be used for staff meetings.
Unlisted threads aren’t really at all secure if anyone has the link they can access those unless they’re in a locked category. Anyway would make sense to not send out notifications to everyone about making an unlisted thread if people want to do that and only share link with some non-staff folks.