You are told when the pirates dig up a buried treasure chest and also when they put a treasure chest on their ship.

Pirates Just Dug Up A Treasure Chest!

The pirates just dug up a treasure chest. They move at the same time as you do. Any buried treasure indicators (a big red X) that has a pirate flag on it means that the pirates have already dug up the treasure there. There is only one treasure buried per red X.

Pirates Just Got A Treasure Chest On Their Ship!

The pirates just put a treasure chest on their ship. They move at the same time as you do. Any buried treasure indicators (a big red X) that has a pirate flag on it means that the pirates have already dug up the treasure there. There is only one treasure buried per red X.