One of many Treasure Hunt quests, there's plenty of loot for completing this little task.
The Witcher 3: Temerian Valuables
If you head north to the uppermost small bridge marked on the map you'll see an Undiscovered Location.
By the side of the river there's a dead soldier who you should search to find a key. Under the water close by is a chest which you can unlock to find a document revealing this quest.
Once read, the document will send you east past the Mill signpost to an abandoned shack. Inside and under the cellar you'll find the valuables, including weapons and armour. It's as simple as that.
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