I bumped a car no damage and left

returned two hours later with a pen, but the car had gone

Really? 2 hours later? Were you manufacturing the pen yourself?
How unlucky you were that the car had gone

A couple of days ago I bumped into a parked car whilst driving into a car parking space. I accessed the car and saw no damage whatsoever to mine or their car. However, a couple of people in cars could have seen the incident as well as a guy on a phone. The guy on the phone said that I should leave note, I asked the guy on the phone for a pen and he said he didn't have one (I didn't have one) so I parked next to the car I tapped and left for work and returned two hours later with a pen, but the car had gone.

I'm worried someone would have reported it as a hit and run.
Does anyone know the laws?

Thats nothing, a few months back, I was attempting a parallel park, before it was too late to change my mind, my car bumped into the car behind. First, their was a small banging noise and I didn't know what to do, so just drove off.

Trust me, you'll be fine. Plus if there were witnesses, who nowadays can be bothered to report something like this, aint nobody got time for that.