Forgot to Turn Off Car Light

If one sees that his friend forgot to turn off his car’s lights, there is a mitzva to inform him to prevent damage to his battery. If he does not know who the car belongs to, he should put in a bit of effort to find the owner, e.g., by knocking on a few nearby doors or the like. However, he does not need to put in too much effort, as that would already be a loss of time or money for him.