The map is in the North-Eastern part of the location, right next to the chest. Approach the discernible narrowing of the land there, turn to face North-East and go ahead.
The map will take you to the Petite Caverne. In the South-Western part of the location, at the back of the smugglers' camp, there is a visible elevation of the ground. That is where the treasure is. Treasure's Coordinates: 901, 263
The treasure: 3000 regals, elite heavy shots storage design plan.