Edit : Remain on scene or leave...Good topic?
In this case it fits in with what he was doing when things became physical. He left the area as what had happened was another attacker joined in.
You could consider expanding that important question to what to do after the last shots are fired? Stay, go, securing yourself and others etc.
Leaving might have been a good way to check himself for any wounds in the safety of the car. It wouldn't be out of the question in this particular circumstance to check for stab wounds or even small fingernail cuts. In the initial physical altercation his adrenaline dump might have caused him to not notice any injuries.
Using the scenario of when dogs attack you would try and get yourself to a doctor/hospital to have a series of shots if you have been bitten.
That is an important point you have raised of what to do immediately afterwards.