Fort Dunstad - Trap Door

[O0mix0O]

Hey, so I was just trekking across Skyrim and came across Fort Dunstad (Directly south of Dawnstar and directly east of Morthal). There is a tavern, prison, captain's area and the actual fort. After clearing all bar the fort I head inside to kill everything inside.
That done I randomly crouch and look under the a 4 legged structure and underneath some ashes is a trap door which requires a key. Just spent about 10 minutes looking for the key but cannot find it anywhere.
Am I looking in the wrong place? Probably. Am I looking at DLC? Probably not.

- Any help would be helpful