Early in the game when you kick the seekers out of the Black Boar inn, if you tell Louis it wasn't a big deal, things just got out of hand, you can talk to them in the barracks. They will be grateful and one will tell you about their commander's loot stash under the church, and that he often forgets to lock the hatch (coincidentally, as soon as you know this the hatch will be unlocked). Whether you loot the place or not, you can rat them out to their commander for revealing his secret.
If you get the seekers sent on a dangerous mission as punishment, you may not be able to get in (you encounter them later fighting a troll, near Lovis' tower; at least in D2:ED if you were fast enough you could talk to them and get the hatch opened).
You need to mindread Derk to get the key for his cellar, which can then be found in the barn next to the Black Boar inn.
