@donkeypong, I believed Brody was random person. But seemed he's a great treasure hunter and he read and listened more about Finn's treasure and finally he started hunting it. He has best knowledge about it.

Brody was a big believer that there would be clear natural features (probably geological, which would be the most permanent). When he saw them, he’d know what they were.

If Brody believe nature and knew more knowledge about nature's features he can find treasure after hard working. Also admire other peoples great human being of him and then others try to provide their known information to him.