He should’ve dug it up:
If John and Mary had found the coins while taking a walk on someone else’s property, the booty would likely go to that landowner. But what if someone stumbles across something valuable on public property? Say a San Franciscan strolling across the Golden Gate Bridge finds a bag containing $1 million in cash. In California, there is a law mandating that any found property valued over $100 be turned over to police. Authorities must then wait 90 days, advertise the lost property for a week, and finally release it to the person who found it if no one could prove ownership. Orth says it’s rare for cities or states to make any claim to found property, like the goods that metal-detector-wielding treasure hunters find on public beaches, unless it has some historical or archeological significance.
https://time.com/10118/california-gold-coins-finders-keepers-john-mary/
I don’t think he has a legal case, but his story is plausible enough that if I were the FBI I would be suspicious that someone at the FBI got his buddies together and stole it. I would want to check it out.
44 Again, the kingdom of heaven is like unto treasure hid in a field; the which when a man hath found, he hideth, and for joy thereof goeth and selleth all that he hath, and buyeth that field.
(Before the FBI can confiscate that wealth!)