That is not a plausible theory. Sure, the treasure might be elsewhere, but I wouldn't keep the prize in my own possesion if I were over 70 years old. What if someone finds the chest after his death.
I would rather believe it contains a note requesting that Fenn should be contacted by the lucky one.
Assure you that there are ways to deal with Fenn dieing and still require the party finding it to make contact with someone to claim the prize; in fact, Fenn planned this when he thought he would die from cancer.
From Wikipedia: "He filled the chest with "treasure" containing gold nuggets, rare coins, jewelry and gemstones, along with a jar holding his autobiography. He intended to hide it and end his life nearby, with the treasure as a legacy.[2] However, he survived his illness and waited until he was 79 or 80 to hide the treasure."
These are all bearer instruments. You may not know if it's found. 79-80 is 2009-2010. If he had a cellphone, there's a trail for some enterprising telcom worker or NSA analyst to follow (base station->sector->signal strength if not outright location). Presumably he drove there, maybe some ALPR database to mine.
Key to a safe deposit box and an address of the bank is more than enough to fulfill that requirement. At my bank, they come with two keys. Put one in the box and keep the other, and check the safe deposit box periodically.
I don't have the time to dig this up, but banking law forbids unknown parties to access safe deposit box in major banks. Sure you can give the second key to anyone you want but when they showed up at the bank, they will have to show their ID that matches name on the list of people you gave permission to access the box (at least that's how it is at Bank of America, Chase and Wells Fargo - I don't have boxes at other banks). This is part of KYC (know your customer) routine for banking institutions.
All he needed to do is leave a will-type of letter in this chest and directs you to bring this letter to his lawyer and that will do it.
You're right, I actually asked a banker after I wrote that and she said you'd need to show your ID that matches the name on the account. Sorry, too many bank heist movies I guess.
At my credit union they've moved to using thumbprints on a reader for regular visitors so the staff doesn't have to go through checking signatures, etc.
I'm executor for my aunt. She has a safe deposit box at a bank to which I have her key but I'm not on the list of people allowed access. They would have allowed me to open the box and remove a copy of the will if that was the only place it existed (that was also the situation with my dad) but otherwise I would not be allowed access.
Interestingly the agreements I've signed specifically disclaim any responsibility for gold, silver, cash and similar items.
I would rather believe it contains a note requesting that Fenn should be contacted by the lucky one.