There are a number of Chest keys in The Witcher 3:
Chest key found on a small island north of Sund
This key opens a chest on a rocky islet near the body where it is found. The chest contains a sword, a crafting diagram, an alchemical formula (if it has not already been found or purchased), some 
and a few crafting components.
Missioni associate
- Hidden treasure
Chest key found on Faroe
This key is found along with a letter.
Missioni associate
- The Price of Honor
Chest key found by the Vikk Watchtower
This key is found along with a notebook. It unlocks a chest at the top of the tower.
Missioni associate
- The Royal Air Force