I'm like Dave and know that the red Yes means there are threads in a forum that I haven't read; the red Yes doesn't bother me. I know that the threads I'm watching will pop up on my home page when something new is posted.
I'm sometimes curious ... OK maybe it's nosy
and when I notice a red Yes for a forum (even though it's not a thread I'm watching) I will pop into the forum to browse and check out new topics/threads.
For instance, I watch this "Site Talk" forum and noticed a red Yes so I clicked on the forum and found this thread.