That says it all.
Also, when I first noticed it, I asked about it on chat, and someone said something like "Apparently, it's had been there a while, but I just noticed it."
Is it intermittently displayed and what determines it?
They were A/B testing it as a new feature. Anna Lear commented on Meta.SE on September 6th:
We're A/B-testing this tracker right now. Assuming it performs well, we'll put more time into "congrats! you earned X" notifications. (Our initial designs for it interfered with how we decide when to mark achievements as "read" and we opted to run the test without making any changes to that first.)
You won't be seeing it again for a while, though (if at all). Thomas Orozco, one of SE's Product Managers, commented yesterday:
The A/B test has completed for now, but we'll probably run another one before making a final decision (for a variety of reasons that don't really make sense to explain here). Either way, if we were to decide to move forward with it (that decision has not been made yet), we'd work towards making it less intrusive. So, for the time being, I'll make this status-review.