Could the Exo Stranger been the 5th person to the team because think she can go in and out of our universe or possibly timeline because she is always warning us of the vex's technology she could have been infused with something evil because her card states she has no ghost and the rumors have haunted the tower for a long time so is it possible she escaped by figuring out the vex's technique to entering and exiting our timeline/existence and she used the tech she had wielded to get her self out of the vault by teleportation like in the mission the Archive I think it was Ghost says "all of their minds are connected to a central mind and that's how they react so fast" so possible she used it to her advantage and teleported her self out of the Vault of Glass just a theory and on the Strangers Rifle ghost says "parts of this does not exist yet" so maybe she travelled back in time from that fateful moment of her team falling to Atheon and the Vex so she could earn you of the vex's power