Walkthrough for the Treasure Hunt Hidden Treasure: Coin Doesn’t Stink in The Witcher 3 Blood and Wine.
This Hidden Treasure event is part of the Side Quest [Wine Wars: Coronata], but it can be completed as a stand-alone quest, as well. If you complete this Hidden Treasure quest before starting [Wine Wars: Coronata], you’ll get credit for it retroactively, so there’s no penalty in doing this quest early. You can start this quest by simply approaching its map marker, which can be found a far distance east of the Coronata Vineyard, just across a bridge that spans the river.
Unfortunately this place is guarded by a pair of Rabid Rock Trolls (level thirty-seven). Apply some Ogroid Oil and use Quen to keep yourself safe from their punches and thrown boulders and you should be fine. Kill them, then loot a corpse near the end of the bridge, upon which you’ll find a Letter to Cosmo Cyrille and a Key to Lockbox. Read the aforementioned letter to start the quest Coin Doesn’t Stink, which will draw you back to the Coronata Vineyard.
Don’t bother running back west, however, as there’s a convenient signpost to be found to the south. Just follow the road south, southeast until you reach a fork, at which take the eastern road to shortly discover the Dun Tyne Crossroads signpost. Use this device to fast-travel back to the Coronata Vineyard and make your way northwest to find a search area. At the southwestern end of the search area you’ll find a pile of dung covered in straw, where the application of your Witcher Senses should detect a chest sticking out. Loot the chest to complete this quest.