Couple of days ago my car was parked in front of another car. Given that I needed more space to get out of the parking spot, I drove backwards and hit the front of the car behind me. I got out of the car and saw no visible damage to both vehicles, apart from the registration plate of the other car being slightly bent. I assume the crash happened at around 2-5mph. The car was no more than 2 meters behind me, so there was no space to accelerate much and of course I didn't press the gas pedal hard. I also hit the brake just before the collision, so I assume the speed was really low.
Since it was late and dark and I was in a hurry and didn't see any damage, I drove off. I also didn't have a pen to leave a note. The next day I realised that this was wrong and felt guilty about it.
So my question is whether it is possible to have caused some damage, given that there were no visible dents apart from the bent registration plate? More specifically, is it possible to have affected the safety of the car?
My car is a BMW 3 2007 sedan, and the other car was an Opel of similar size (probably an Astra) produced in about the same year.