Mission 10: Bad News
Mission 11: The Order
When you pass through the server rooms, you will eventually come to a divinity statue down some stairs. Go back to the left and left again to find the key there behind some glass. Smash the red jewel to the right of it to gain access and collect it.
To get this door, you need to be pretty quick on the feet. When you enter the servers rooms, take the left path and go forward down the hall. When you punch through the wall on the right with the red jewel, quickly run around the corner and there will be a metal door on the left that is closing. Run in before it closes and you will be able to access the secret door within.
Mission 12: Under Siege