You can capture landed and crashed UFOs by clearing them of aliens and keeping at least one soldier inside for five turns. What happens to any aliens and/or items left outside when the mission ends that way? Are they recovered?
They all will be recovered. You have controlled the crash site, so what happens is the remaining aliens escape and you suffer a reputation penalty for letting them escape, but you control the crash site and therefore keep everything around the crash site. Think of it from the opposite point of view. When you retreat from the crash site the aliens maintain possession of the items (until the military takes care of them, but whatever) so when they retreat from the crash site you maintain possession of the items. This should be verifiable by checking the post combat screen to ensure that items are showing up on that screen that weren't inside the downed UFO.