Is a sidequest in Novigrad.

After talking to the bandits, they wish to find their "friend". Use witcher senses, to find the initial footprints etched with sewer slime.

Follow the scent, and you'll come across blood against a wall. Keep following the scent, and you'll come across a wounded witch hunter.

You can choose to accept their money and leave the witch hunter to his own demise.

When the second part of this quest opens up. Head to either of the available map markers to find the keys.

One location has a dead body, and a torch that is used as a lever to open a secret doorway, the opened area contains the key and the locked chest. The locked chest requires all the keys to open.

At the other location you will encounter a woman who says her house is haunted. Choose to help her, and follow her to her house. It turns out witch hunters were waiting for her. Defeat these witch hunters and she will give you the final key.

Return to the previous house, use the keys to open the safe.