New reply notification - what is it checking?

[opentype]

 

I have old forum topics, that when kept open for while, report all already displayed replies as new replies to load  – even though everything was posted many years ago. 
I’m trying to get to the bottom of this but I just can’t find the reason. What is the query that is executed in the background?
I would assume it checks the current timestamp against the timestamps of posts in that topic, but the post timestamps all look okay in the database and are way in the past. 

[bfarber]

It might be that the first post in the topic is not flagged correctly, possibly. Typically the topic can still display normally, but other oddities can crop up. You'd want to look at forums_topics.topic_firstpid and forums_posts.new_topic.