title explains it all
if you are confused:
if you set some categories to watching automatically, then every topic ever made in that category is set to watching. This includes unlisted topics as well.
how to reproduce:
go to preferences, click categories on the left hand side, and then set a few categories to watching. after that, you can see any unlisted topic that someone posts in.
cool pictures lol:
i have never been tagged in this topic, never have posted in the topic, i just got randomly tagged. the same happens to other topics.