It takes a little time for something like a "shot" to register. By the time we figure it out, I'll betcha it's a couple of minutes.
With the cover of the greenspaces around these areas, the shooter merely moves "totally" behind his screen after the hit, places his weapon in a backpack and walks away to the NEXT parking lot over to a waiting vehicle OR just keeps on walking.
What we can guess is that he has been to all the sites before and we should start prodding the public (like a groundskeepers)about someone unusual in the area...perhaps a few weeks back.
The sniper has engaged in a game...the part he enjoys the MOST is the "ritual" prior to the shooting...he is very organized and sees his "ritual" as confimation of his superior intelligence...the END RESULT isn't the death or maiming...the RESULT he is striving for is the media and police reaction...BECAUSE if he were really out to KILL people as his primary goal...he would fire two or MORE shots...or use a more powerful weapon.
It is a deadly game...and it will only stop when he becomes the loser...he will most likely take his life when surrounded, because he will NOT want to be the loser or be held accountable and "let" the police win.