While exploring the Lansdowne Residence you'll find a locked chest in one of the bedrooms. Examining the chest will begin this side mission. The key to the chest can be found close to the end of this chapter so after obtaining it you will have to backtrack quite a lot in order to check its contents.
The key needed to open the chest can be found close to the altar in the Fraternal Order of the Raven, next to the first Infusion. Once you've picked up the key go back to the Lansdowne Residence.
Backtracking to the chest is a profitable action, because the chest holds valuable treasures - a new Infusion and a bottle of salt.