I put that there to illustrate the glass house, Sherlock. It isn't realistic to expect every bit and shard to be documented. Park rangers are some of the worst for keeping bits of historic sites where they work. Hopefully they don't keep very big and important pieces, just trinkets. I've worked in places like Mammoth Cave in Kentucky where it wasn't the rangers, but the speleological society folks, with permission to explore passages, who used to leave the caves with garbage bags full of loot from prehistoric cave users.

Here in the US amongst some folks who work in public recreation areas, those metal detectors are considered an amateur shortcut to chunk change that ended up on the beach instead of in the couch cushions.

SRS