I think you're right. Because at first I'm wondering how on earth they knew this. But I bet this Asian fellow saw nothing. What nut would see a man get shut and drive off? Even a gun shot in this day and age?
Let's see.....a guy in a gas station, no witnesses, shot in head, dead. That the weapon was a shot gun being bandied about but as I recall that school kid was at first said to be shot with a handgun. I'm no gun expert, but I don't think you can tell or shouldn't deduce, right away, what kind of gun was used on a victim.
Even if it was a shotgun, yeah, the sniper could have changed guns. This doesn't yet look domestic or drug related. It looks suspicious is what I'm thinking.
Someone in fear for their life. Someone gets shot within a few feet of you, are you just going to stand their like Steve Martin? Fight Or Flight reaction, and he chose to flee. Now I might decide to return to the scene in a few minutes after police arrived, but the guy may have been in shock. Of course it could also be an immigrant, perhaps even legal but wary of police/gov't officials. Not at all uncommon among immigrants.