I would like to think the smile would have galvanized my anger and I would have made a scene; maybe pointed at him and run at the car screaming, "child molester!" I've heard when a flasher flashes one is supposed to point and laugh; anything that would be not something he would like/expect would be what I'd do.
One does not remain a child forever but one does remain a child molester so I'd feel like I had the upper hand; I could remember I was no longer a child and act on it but he couldn't remember not being a child molester.
I avoided my abuser the next 3-4 years he was "near" me, never saw him as an adult. I did dream about him once a few years ago, in the dream I was taking my stepmother to where he was (large crowded party or group thing of some sort) so they could meet up again (she liked him; did not ever know he abused me).