I’m not too sure, myself. One of the many annoying things about these new forums. I think someone made a tool to check it, but since I can’t quite find that, there is this thread with further instructions:
Aww, I’m sure you haven’t lost it. Vanilla was chock-full of little secrets to find. You probably just found a very unique and rare one! Luckily you will be able to discover it again soon when Classic launches : )