Treasure Hunt: Deserter Gold is a Side Quest in White Orchard. This quest is located in the far North, to the East of the old Mill, and lies on top of a ridge near the very end of the map.

Deserter Gold Walkthrough

Here you will find a dead body, and a small camp (and possibly a large pack of Wolves and Wargs - so watch for ambushes). Upon inspecting a nearby container, you will find a note from a spy, who saw the group deposit a large cache of valuables in a nearby abandoned building near the mill.

Head South and travel to the mill, where you'll find the very same abandoned house with a secret cellar. Hop down and force your way in to find the large assortment of chests - all with very good randomized loot. Note that the very last chest is locked, and you will need to do the other hidden treasure quest: Treasure Hunt: Temerian Valuables to find the key.

Also note that if you have already completed that quest, reading the spy's notes will not start this quest.