Sometimes the forum gets drunk if you use your phone sometimes. Mine likes to mix up everyone's avatars and it uses a picture of a marine sunfish for a lot of the sidebar images like the main logo and the speech bubbles with ellipses in them.
It's a speech bubble with quotes inside it. I have a feeling it's a third party thingamy to do with a plug in, but I'll admit it's been a while since I've seen one.
It's odd that you haven't been seeing it. LOL!
That icon when grayed out means you have read that forum, that icon colored in means there are new posts you have not seen in that forum.
So in the screen shot above you can see some forums have unread and some don't.
Using tablets, phones, etc. is probably why it's not showing up, or depending on the browser.