There is no key. 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.
Even if you get the seekers sent on a punitive mission, the next time you see them (fighting a troll near Lovis' tower) if you are fast enough to talk to Merrill before they die, you can still get the quest, unlocking the hatch (presumably an oversight in the dialog system).
