When going through the Deadmines, there is a locked door before Ironclad Cove with a cannon pointed at it. If you don't have a rogue or engineer or a skeleton key, you can go into the small alcove you passed before arriving at the cannon and loot the gunpowder from a small Defias Gunpowder barrel. This allows you to fire the cannon which eliminates the door and spawns an elite patrol (which can take a group by surprise if not careful) to investigate the noise from beyond the door.