This is embarrassing, but maybe other people have missed it too.
I only recently started paying attention to the little circles on the far left of the main page. When they're blue, it means there is something on that board you haven't read yet. Duh.
But wait ~~ there's more. If you open the whole board, instead of just the thread showing on the main page, there are other blue lights that say "new" next to any threads with postings as yet unseen by you.
I was so much in the habit of looking at which thread headings showed in the far right column on the main page that I missed the much easier to use blue circles on the far left. Even after I started using them, it took me a little while to catch on to the "new" doobies.
There they were, right under my nose. You'd think I'd investigate, right? But nooooooooooo, not me.
Spindrift, the gray circle indicates you've opened that board since anything new was posted on it. The blue circle means that something new has been posted since you last looked. TAKE NOTE: if you open only one thread from a board, it will then show you a gray circle on the main page, even though that board might still contain postings new to you.
That's why it's good to open the whole board, not just the thread showing on the main page. That way you get to see how many threads have the little icon next to them.