I too think that the sniper is using the van to conceal his aim & shot. It would quiet the shot & provide a quick get away. The green spaces likely provide a long unobstructed shot and low foot traffic at the shot site. The bullet traveling through the air does make a sound, directing attention back to the origin of the shot. The green spaces would keep witness ears away from the line of fire. With all of the talk about a white van he would be stupid to keep using it. He will likely switch vehicles without changing his M.O.
The police should conduct an exhaustive search of all mini vans with a vin number indicating no passenger window within 10, 20, 50 miles. They should also post a police or volunteer concealed watch on all prime green spaces.
The shooting at the Bowie school...they found where he was laying (he plastered the grass down) and also found the spent casing and note not too far away...that just BLEW the shooting from the van theory...for at least ONE shooting site.