I refer to the aliens that die when shot down on the Geoscape. With version 22.1 and 22.2 when I breach a damaged UFO, it's filled with the corpses and weapons of the crew that died on impact. It also doesn't show up when I salvage and sell the weapons after the mission. The only weapon salvage I get is from the aliens I personally kill in ground combat.
Edit : After reading around a bit, I think this is related to the fact that only three spawn regardless of what kind of ship being shot down.