You have probably done all that is necessary.

A few years ago I was in a similar position; I grazed another car in a multi-storey car park, causing slight paint damage. I left a note with my details.

I never heard a thing.

If the weather was bad (rain/wind), the note may have either blown away or is now unreadable, especially if the vehicle owner (like me) doesn't use their car very often - they may not see the note for days.

If you aren't that far away, it may be worth (to settle you conscience) to revisit the scene when you have more time and go door-kocking armed with your photos on your phone as proof as to what occured. Hopefully the person affected will take that as you being a good citizen and not try to escalate things into some confrontation. Keep to the facts as you saw them though, as some people (sadly) these days may try it on to get other damaged repaired that had nothing to do with you, a-la crash-for-cash.

A former neighbour dropped a broom (or suchlike) when loading his things into the moving van and broke my car's rear brake light, but he left a note - admitedly he found out whose car it was and posted the note through my letter box, even including the information he'd got from my local dealership with a quote for the repair and offering to pay in full. Which he did. No insurer involved, everything sorted, him £150 or so lighter.

You may find if the damage was light that they may not be bothered (especially if it's an older car in not that good nick to start with, or that the scratch can be removed easily if it's just transfer from your car and especially on plastic [no rust worries]). Really up to you how far you want to take this to clear your conscience.

I would follow up, but then I'm a mug for such things and would'nt be able to let it lie until it was resolved.