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.
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.