I often wondered about stories like that, but also the possibilities of what may have happened to some or most of the loot. You have to think in todays times for way back then. It would seem there would be dishonest and corrupt people back then as they are today.
Let's say a robbery or gold shipment was looted. Robbers on the run with the loot. Sheriffs, Marshalls, or a small posse goes out after them. Lets say even it is 2-3 robbers and the Sheriff and only a Deputy, (providing the Sheriff didn't go alone), goes after them. Sheriff located the robbers, gunfight ensues, robbers gets killed. Now it is only the Sheriff or/and his Deputy. The Sheriff says, "We don't get paid much and we can split this loot and live pretty nice!" "We can say we killed the robbers in the gun battle and never found any of the money no one will know and we can just act like we looked around for the loot but without success!" If the Sheriff goes out alone who would know if he didn't pocket the money after he killed the robbers? You have to think about how the times were back then and how people lived. It's kind of like, if you were getting paid $20 a month lets say, you went out after some robbers who stole $500, you found the robbers and killed them in a gun battle. Would you turn over $500? Keep half of it and say that is what they had after you killed them? Or keep it all and say they didn't have it and you never found it? $500 back then was a lot of money!
I have wondered about most of the lost loot and stage robberies. I also speculate that some of the robbers probably spent a majority of the money when they did steal it if they got away! But over time, some of the stories could be exaggerated!