I've played through this mission twice, each time going into every room (even checked the roof where the dropship is) and I cant find the last alien to end the mission. Each time I've aborted, and it shows 1 alien still alive. Not sure if there is a fix for this, but I remember a long time ago reading about a command you can type in to summon an alien in front of you if they are stuck in a wall or something.
This is the bomb shelter mission, the one with the gas truck in the middle of the map on a platform.
Any help would be appreciated.