To obtain this trophy, open all chests in any one cache location. These locations are marked by a treasure chest icon on the map. On the map, you can also see how many you have obtained in the region. The chests are not very difficult to find, and most of these locations only have 2 to 3 chests. This will likely come naturally as you progress through the open world. There is no trophy for opening every chest in every cache location in the game!