A: How was the fox caught? What traps, what methods and by whom? You or your trapper.
B. Missing head on a found bird is not characteristic to a fox. Coons and owls do that. I wouldn't let my guard down just yet.
C: The dug hole under the fence is a great educational opportunity. Animal started dig at the base of the fence. So put an apron there....horizontal wire laid flat on the ground.....and they will hit that first, and fail. May cast up and down the fence and try again, but do not back up the width of the fence apron. Far easier to install than to bury the fence in a dug trench.....and just as effective.