Sounds like the guy removed the gun from his waistband so he could stuff store merchandise down his pants, set it down and then forgot to pick it back up before heading out.
Then again, SOB or not, [early onset] Alzheimer's could account for it.
Recently saw that story line in a cop drama with an aging detective.
OTOH, autopsies on relatively young drug addicts have shown the same grey-cell deterioration as older Alzheimer's sufferers.