What is the ruling regarding someone who rented a car and found a slight scratch on it when returning it, and does not know whether he caused it or not?

The default is to ask the car owner. If he informs you that the scratch occurred before your rental period, then nothing is incumbent upon you. However, if it occurred during your rental period or he does not know when it occurred, then the default is to attribute the incident to the closest possible time. If it is proven that it occurred during your rental period, the renter is not liable for damage except in cases of transgression or negligence. If there was no transgression or negligence on your part, then you are not obligated to compensate. But if there was transgression or negligence on your part, then you are obligated to rectify the defect.