Airpods Not Showing Up on Find My Iphone

I misplaced my Airpods and I went on Find My iPhone to check for their last location. I've used Find My iPhone before to find my Airpods but now they're not even listed as one of my devices. It's disappeared like I've never even owned them.

I'm confused as to whether this means my Airpods are dead or if someone has taken them and that's why I can't see them anymore? What are the anti-theft features on Airpods that could help me in this situation?