that is exactly what i thought when i heard of the shooting as well. That school was my polling place when i lived over there and the lot is always deserted, and you are correct, completely unlit.
As an addenum, my brother does work out in Gainesville, and knows the Sunoco where the Manassas shooting took place.
He drove by several days ago and did some observing.
I don't know that area very well, but he said there is a Petco (?) and a Bob Evans restaurant directly across the street (Rt. 234?) from the Sunoco. He drove up to the restaurant and noted that between those two buildings is a driveway providing a perfect line-of-sight into the Sunoco. Since that shooting was at 9:30PM, he thought it would be quite dark in the driveway.
So it looks as if the shooters have a consistent MO, and they select their position very carefully.
What boggles my mind is that the police have this information from ballistics evidence (that is, the line-of-sight, shooting direction and probably distance) and they say nothing about it to the public.
Seems to me they should be warning to public to report any strange sightings of vehicles pulling into darkened across the street from shopping centers, gas stations, etc.