In the USA things are still somewhat a free for all. We are not necessarily under penalty of law to reveal anything found on private property.
In the UK they are bound by law to reveal any coin find over 15 in one hole, silver coins, and any gold items. I don’t know all the particulars. In the US private property is generally finders keepers unless you are trespassing, and technically government property is prohibited, although many city parks and beaches are free game for us.
Should've given the guy SOMETHING.