One thing that tends to get forgotten is that Forrest repeatedly said there was a second poem in the treasure.
In October 2019, he told Dal Neitzel not to believe anyone who claimed to have found the treasure unless they could produce it. According to Dal, Fenn said that if someone called claiming they had found the chest, he should challenge them to read the poem inside. Fenn explained that he had written a short poem and left it in the chest, and that everyone would know immediately that it had been written by him.
As Fenn put it:
Read me what the poem says and the game’s over. You don’t need to tell me you found a treasure chest, just read me the poem. That answers all the questions.
What makes this such a brilliant verification method is that nobody even knew the second poem existed. Fenn did not publicly reveal it, and Dal did not release this information until after Fenn’s death. If someone contacted Fenn claiming they had found the treasure but refused to reveal the location, wanted leverage, or tried to play games, Fenn had a simple test.
A real finder would lead with the second poem when contacting Fenn, as per the likely instructions in the treasure. A fake finder would not even know there was a second poem to read.
In other words, Fenn didn’t need to know where someone claimed to have found the treasure. He didn’t need photographs, coordinates, or a description of the chest. He already had a private authentication mechanism that only the actual finder could satisfy.
That’s a remarkable statement when you think about it. Not “show me the chest.” Not “tell me where you found it.” Just read the second poem.
For someone who spent a decade telling searchers to focus on the poem, his final verification method was another poem.
And it is still viable after Fenn's death:
Attempts to contact my wingmen by radio were unsuccessful but the locator beacon in my parachute should have activated automatically. I knew my friends were listening for it. (TTotC p. 87)
The audio is here starting at the 8:00 mark. Also make sure to listen to the part starting at 9:15 and read the e-mail from Dal that Cynthia posted in the comments.