Hidden From the World is one of the many treasure hunts you can go on in Velen.
Note that the rewards can be randomized, so you might not get the same loot as the one listed below.
Following contains quest spoilers. You’ve been warned
Find the body.
Across the water, southeast from the Ferry Station signpost, you’ll find deserters in a camp overlooking a road. There are three deserters, all are level 9. Kill them. You’ll find the corpse lying between a box and two barrels.
Look for the chest.
Go south to the objective area. You’ll find the chest hidden among some bushes.