[TrainedMonkey]

Or he has access to aerial surveys that indicate that there was no digging at the site.

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.

[solarkraft]

GPS tracking? Some kind of tamper-detection with a sender? At his budget he could be communicating through satellites.

[nerfhammer]

would need a permanent power source to do that. batteries slowly leak power even if unused. with a solar charger batteries would eventually wear out from the charge cycling. GPS and satellite communication is relatively power hungry. plus, the radio emissions would be a dead giveaway.

[solarkraft]

Good points.

[nogbit]

In the chest is 40lbs of lead and a key with a note. The key is to a safe deposit box and the note says what bank and where. He also has a key to the same box and can check that box whenever he wants. He knows it's found when the real loot in the safe deposit box is gone.