Bodies disappear too easily/quickly after you’ve killed them. It seems as though, if you got a certain distance outside of the dead body, the corpse disappears.
This is a problem considering all of the moving you have to do in order to survive. This is especially true of quests that require you to take on multiple enemies by yourself.
I suspected this to be the case for sometime as I was often unable to find bodies after a very long fight. In the end, I just thought it was me but while completing the quest Interloper, I killed the leader first and had to keep moving around to kill the rest of his men.
When I got back to the camp, where he was, and where the quest marker for his corpse still is, the body is gone and it now appears that I cannot complete this quest since I don’t have a save from before I killed him.