I mean, the finder would probably think "best not tell people where I got this from" a la Counte of Monte Cristo.
Here is a fantastic podcast describing Fenn’s story:
Or the treasure is of the form "take this letter to the bank in order to get your reward".
Or maybe he is confident it is not found because it can't be found because it is a hoax.
This is explicitly declared in the fine article. It's one of the clues referred to in the title.