I do some surveying for my civil engineering job and use a metal detector to locate the iron pipes that mark property corners. I often find nails and barbed wire before I find the pipe. There's not much to be found of value in California. The truly ancient stuff around here would be left behind by people who didn't have metal.
I'd love to find the REAL plate left by Sir Francis Drake.
https://en.wikipedia.org/wiki/Drake%27s_Plate_of_Brass