A thought on this:
There's actually some stories of this very type of thing happening in the decades before metal detectors. And thus, the persons were forced to look for their items by eyesite only. For example: There was a vending machine company that was robbed of sacks and sacks of coins (you know, those burlap type 50 lb bags full) .......... in the late 1950s, in a city near me. The cops were able to respond to the crime scene, just in time to see the vehicle fleeing the scene. So they started pursing. It ended in a chase, that led out of town, to a mountainous curvy road. The car lost control around a curve, and crashed down an embankment, scattering coins all over kingdom come. The only thing law enforcement (and the vending machine Co.) could do is rake and eyeball through the vegetation. After a week of such backbreaking work, they figured they'd found as much as they were ever going to.
The person telling me the above story, in the late 1970s, was a kid in the mid 1960s, when he learned of that story. He said that he and his friend would go to the site, and rake through the foilage, even though years had gone by by this time, and sift, rake, etc... through the roadside gravel. They could still get several dollars in change for their effort WITHOUT METAL DETECTORS. A few dollars in the mid '60s was still worth someone's time to work for I guess. Can you imagine what a detector could find at a site like this? It would be child's play to finish up finding the rest.