SMF unread tracking system

[rawonam]

Could someone tell me how it works from inside (what information is stored in the DB tables for this purpose) or where I can read about this system?
I'm using SMF for a few years, it's a really convenient feature. I've noticed, tough, that sometimes really old topics get marked unread, probably there is a limit of tracking time, I don't know.

[onepiece]

Data is kept in log_topics table and all mark read functions are in Subs-Board.php file.