Where: Shipwreck in Blaustein
What: The key to the Captain's Cabin you pick & the Captain's cabin.
You can unlock the door to the cabin. However, if you rest the door re-locks and it cannot be unlocked. Also the key disappears from your inventory.
Steps to recreate: Do the dungeon. Then rest in front of the Captain's cabin after unlocking it (but not opening it, just unlocking it)