A guy contacted me years ago about looking for an old castle tunnel in France(?) iirc. The family heirloom shotgun had been passed down to him. He was taking it apart to have the stock redone and found inside the long tube inside the stock a rolled up treasure map!
It was some relative that lived in an old castle and had fled with the Germans came in. They escaped right before they came and had stashed guns, money, gold, etc. in the hidden tunnel.
I forget why the relative never made it back to the castle or why it didn’t stay in the family. It is now part of a national state property. I know he was trying to work with lawyers to let him on to the property to explore - but didn’t want to divulge why. He kept on getting refused.
Interesting, if a little sketchy. :^) He needs to scan that map and lock up the original, with a cover letter describing the whole process. He may have to get some help with this search, maybe from the French gubmint.