Hello, I have the same problem, I have no spawn for the third key. This mission is bugged : I had to speak to a guy at the sanctuary, but I didn't do that but the mission has continued with the second key to find at Saero. (I didn't find the first key, I had this, idk why). Now, I can't continue...
Well, We have to kill the enemies for having the last key... I finished the campaign and I won the rewards.
