First, you don't rent a car in your town. You rent a car when you fly to another town and need to spend a few days there. Second, you may forget something like a water bottle or something insignificant in a rented car because you will check thoroughly before returning usually. You won't forget a dead body accidentally taken out before returning a car; "Ops, I forgot a dead body in a car.".

Third, I don't know anyone in my social circle where he or she had a dead body in rented car. I dont' know anybody who knows anybody who rented a car and forgot that a dead body was in a trunk.