What if the White truck is a ruse?
Cleverly designed to distract attention away from the true shooter...
Police stop the truck. Find no gun. Assume they have the wrong truck...
Or alternatively, the shooter waits to hit is target until there is a white truck in the area. White trucks are as common as pedestrians...Wouldn't have to wait long in a hiding place before one meets the other within range of the sniper. Again, everyone sees the white truck and focuses on it while the sniper escapes.
A little slight of hand?
Scott