Say the truck passed him going the opposite direction, then turned around, and came up on him from the rear (which I hadn't considered), then the casings positioning in my opinion becomes even more suspect, as handguns eject, most of the time rather strongly, to the right. So in this case, he would have had to be standing close to the front of his vehicle for the casings to group together toward the rear, and the truck would have had to been traveling extremely slow for the casings to group together as indicated by the markers.
The alternative direction, the truck passing him in the same direction of his travel, and then turning around and coming back (which is what I had assumed) would for the most part necessitate firing during the approach to his vehicle, with the spent casings ejecting and rolling across the road toward his car.
Or, he could have been standing behind his car for cover, suspecting trouble from the truck that just turned around and was heading back toward him.
To my knowledge, the casings, and who they belong to, have not been identified.