I have observed that sometimes, when I open a topic that I have visited previously, but which has new posts, I get an an unread count, even if I am not tracking the topic - today, this happened when I opened the topic in a new tab, but I have also seen this if I visit a topic on mobile but navigate back quickly.
The behaviour seems like what is described in this old bug, which is supposedly fixed.
Example screenshots:
Main page after opening two links in the background:
True. I have to say, I only noticed it on mobile and online when navigating back and forth quickly, before. Having it appear on Desktop is a new thing for me.
Also, I just had it happen even for a topic that I did not visit before:
