Outstanding suggestion, which also indirectly begs the question as to why the OP was hunting with a bullet/load of unknown zero.
We zero under controlled circumstances so as to rule out other variables that might cause us to draw the wrong conclusions, not in the field where the results may be influenced by things like unknown distances, buck fever, rest stability issues, pushing/pulling/yanking/rushing/anticipating shots, etc.
When I first got into hunting, if I missed a shot, I would stop by my range and verify whether or not the issue was with the gear, or with me. Only once did I discover that something was actually wrong with the gear (loose scope). All the other times, it was me. Later, when I started to record my hunts, I could see exactly where I was aiming at the time that the shot broke, see how steady I was or was not, and usually determine what went wrong with the shot.
All these considerations aside, I will add that there is one other option that nobody, including the OP, is considering. This is something I have witnessed numerous times in real life and can even produce a video or two if anyone needs proof. There is the possibility that the bullet actually did hit the hog, passed all the way through the hog and was slightly deflected during the pass through, resulting in the bullet impacting in an unexpected location on the ground behind the hog. The hog didn't fall over dead because the bullet failed to cause any direct or indirect upper central nervous system damage and so the hog ran off.