@Lisa Y, I hope I am understanding correctly.
When the "balloon" is solid it indicates there are posts in the thread you have not read.
After you have gone into a thread then leave it the "balloon" turns grey indicating that the thread has been read.
When you look again and see the "balloon" is once again solid it indicates that there is a new, unread post added to the thread.
If the balloon has been replaced by an arrow it indidates that you posted to that thread; the color of the arrow will indicate if you have read all the posts or if new posts have been added.
IIt might helpful to you to go tn your settings, preferences, linear layout. There you can check the box to "Jump to first unread post in a topic". When that feature is turned on when you return to a thread you will automatically be taken to the first unread post.
What is good for the goose today will also be good for the gander tomorrow.