Might have fallen from the fingertips of a tired or lazy Thomas production line employee that either didn't see where it landed, or couldn't be bothered to simply dig it out and reuse somewhere else?
If it hasn't caused a rust spot to form from rubbing the walls of its captive position over the years and miles the bus has travelled, it probably won't hurt to leave it there.
It's also likely there will be bare or primer metal under it from being sprayed over from the factory.
If it was me, I'd pull it out and discard it. It would bother my OCD side to leave it there not serving any purpose for which it was intended.
Good luck, and let us know what you ultimately do with it...