Dishonored: Security Room Key
Once again this key is on the other side of the locked door. It's not far but bewre as it's pretty well patrolled
Go past the first light wall on the first landing off the main foyer. follow that corridore to a T-junction and hang a right. At the end of the hall there is a door, go through it to a service way that goes off left and right. Take the right passage and the first door on the right is the security room. It has the key and a bunch of pickups. The locked door opens into a service room that lets you re-wire or shut off the light wall.
Be careful because there are guards and the odd maid wandering around.