There are multiple Chest keys in The Witcher 3: Wild Hunt, including one in the Hearts of Stone expansion.
Chest key (Ard Skellig)[]
This key can be found on a corpse at the hidden treasure marker on the shore directly north from Sund, and opens a treasure chest on the nearby rocks.
This is directly behind where Birna Bran is chained if she is exposed during King's Gambit.
Chest key (Faroe)[]
This key is found along with a letter on one of the bodies of Ulf and Mikkel. It is necessary to retrieve Agda's dowry.
Associated quest[]
Chest key (Vikk Watchtower)[]
This key is a quest item in the Hearts of Stone expansion, found along with a notebook at the hidden treasure marker at Vikk Watchtower.