Witch Hunt DLC, Far Harbor Quest
This quest will be triggered or can be aquired during What Atom Requires quest.
Related points of interest
Stages of "Witch Hunt" quest
1. Read the High Confessor's Note
The High Confessor's tasked me with investigating a member of the Children. I need to check his note to see who I'm looking into.
2. Search Sister Aubert's Bunk
It would seem the High Confessor suspects Sister Aubert of treachery. Time to see if there's any truth behind his fears.