Not every heretic hides in back alleys waiting to be caught by a Witch Hunter. Sometimes more wide-reaching methods are necessary.
At least heretics are easy to find. No true son or daughter of Sigmar would be frightened of a pious group of citizens.
Tome Unlock: Converse with a frightened citizen in Grimmenhagen. He spawns in front of the house just north of the circle and immediately turns into an accused citizen once you click on him.
This tome unlock contributes to the Stirring Up Trouble pursuit.