There is a ghost on the tippy top of the tower you climb in the side quest for the exo guy in the tower (his name escapes me but it starts with C). The mission with the ship drive, you fight a ghost of Oryx or something as the final boss. Before you get to him, climb to the top of the tower before you jump across, look up at the antenna and there is a ghost on the spire, I think it is "old Russia 3".