as far as the purple or yellow icon goes it will be from whatever style you are using.
So I am not sure what they are for without viewing the page.
It could be a sticky topic or a topic that has a lot of views or replies.
Or a topic that was rated.
But anyway an announcement post will make the announcement icon appear.
Edit the two posts and select:
"Make this topic a global announcement."
You probably just overlooked it .
