I've been using the same save a lot as a start point for some debug runs... it's got a downed ufo on it, so each time i restart I go and clear the same mission.
Usually there is one sebillian noncom to kill and move on... on this occasion, the mission loaded and after my turn, it immediately ended. All the aliens were in fact already dead. There was no allied npc turn, so it wasn't them that killed him, the mission ended immediately after hitting end turn.
not a shot fired.jpg
not sure if this is by design, but having to load a mission to not do anything feels like a waste of time, even if the mission load time is reduced in the final version