Given that I live in the city, my kids often play on the sidewalk in front of our house. The other night, my 3-year-old son was out playing and, in the blink of an eye, said, "Look, I'm drawing," which meant he was scratching a car with a rock. Horrified, my husband and I took him right inside and explained that he can never, ever do that again. The scratch was very small and looked typical of what happens to cars when they park on city streets. Nonetheless, my husband wrote a note explaining the accident and leaving our phone number. In the rush of dinnertime and bedtime, we forgot to go back outside with the note. In the morning when I went to put it on the windshield, the car was gone. What obligation do I have to find the owner? If the car parks on our street again, should I still leave a note?
This is a great wake-up call that, indeed, in an instant, anything can happen when kids are involved. Luckily for you, it was damage to property and not a person, but your son could just as easily have run into the street when you weren't looking.
Writing the note was the right thing to do, but you would have been better off putting it on the car immediately. Given that you didn't, I don't think you have an obligation to find the owner. If he or she had come out to the car in the morning and been totally irate about the scratch, s/he may have even come knocking on your door or left a note near the parking spot asking for a confession. That didn't happen, so you could say that you lucked out.
If the car parks on your street again, I would suggest not leaving the note. I'm no lawyer, but it seems like you open yourself up to other trouble when some amount of time has passed unless you took pictures the day the scratch happened. For one thing, you could misremember the car and invite someone else to blame a scratch on you. For another, even if it's the same car, other scratches could've happened in the interim, and you don't want to be held responsible for, as you rightly describe, the normal things that happen to cars in the city.
If you hear neighbors talking about a scratch on any of their cars, you could try to figure out if it's the same car. Again, you want to do that in a way that won't make you responsible for everything that's happened to anyone's car on your street in the past month. If at any point, you are sure beyond a reasonable doubt (it sounds good, but again, not a lawyer) that you know whose car your son scratched, don't try to avoid that neighbor. Instead, you could say something like, "I think I know how that may have happened. We tried to leave a note, but the car was gone before we could." And, perhaps, drop off some cookies or a bottle of wine the next day.
By the way, if someone wrote to me with the reverse question — "A young child on my block scratched my car. It's really minor, but even so, how do I get the parents to pay for repairs?" — I would have answered this: By parking your car on a crowded city street, you open yourself up to a wide range of minor damages, ranging from scratches to bumper bumps to side-swiped mirrors all the way up to break-ins. These are some of the things that come with city living. Yes, the parents should have been watching their child more closely, but it doesn't make sense to ask them to fix an insignificant scrape that could just as easily reappear the next week when someone walks by with a key and a bad attitude.
So, take this incident as a good opportunity to teach your son about other people's property. Put your guilty conscience behind you and focus on making sure nothing like this happens again.