I just got a quest from the Konungr of Dorestad to find a robbers' den but I checked ALL of Frisia at least three times and I can't find anything. I even checked the islands, the coast of France and the barren chunk of land between Denmark and Frisia, but there is no one there. I haven't even seen any robbers patrolling around a robber's den, otherwise I would've just followed them.