Just noticed, when a topic gets split and part gets moved to another board, the recent posts page shows the locked moved topic, and when visit it, it will still show in recent posts the next time refresh.
You have to mark all read, or select it then hit mark selected read.
Noticed it when came and saw this in recent posts page:
SMF discussion was Re: Apostrophe in email address causing error (https://www.simplemachines.org/community/index.php?topic=585607.0)
Can't recreate.. it worked just fine here a second ago
Well, the moved topic is a real topic with a real post so it will show up in recent posts, and it'll show up in unread - but if you visit it as normal (and get redirected) it should mark as read as it goes.
When I had refreshed after opening the topic, it was still there for me, until I selected it and hit the mark read button.
Never seen that happen before, but might have to keep eye on future move/splits.
Chances are it was just cached - this site runs on a lot of caching because big site.