Note that these aren't exactly my own gameplay screenshots, but I have myself encountered this exact encounter in the game. Here: http://imgur.com/a/wh0EV Note that the weapon depicted is unique - they literally leave one there on the thing along with 400-500 of the ammo like a player-made mod. Not even a single piece of information to justify it. A UFO crash, and an alien to kill, with a unique weapon to loot. That was the whole encounter. You can't even enter or interact with the crashed UFO - not that I would have wanted to, anyway. Deal with that, as you will.