Good deed or me just being a busy body?

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I was on my way out shopping with my sister earlier and she'd just parked the car and we were sorting our stuff out. I saw another car in the next row of cars ahead of us park and when the lady got out the wind caught her door and it hit the car next to her, she got out and had a good look at the other car where her door hit, and then went to walk off. She came back a few seconds later and got back into her car and drove to another car park in the next row ahead again. I waited until she walked off and then I went and had a look at the damage to the car. She'd left a rather large scratch and DENT! I went into the store and bought a pen and left a note on the dented car, explaining what had happened and left my phone number. Now what would you have done in this situation? Ignored it and gone off to do your shopping or let the owner of the dented car know what had happened??