That flag was definitely Mexican. I have a neighbor across the street with a Mexican flag on their car, I am used to seeing it, my brother in law is Mexican and he has the same flag as well. It may be possible that this vehicle had that sticker on it when it was purchased, I would believe that this person has not owned the vehicle that long, because it has a 30 day tag. This flag could have been on the vehicle when it was purchased.
The van had temp plates I would not make to much out of the flag or decals... Probably the prior owners.
Any thoughts from any of us without inside information are just speculation, and yours is as likely to be true as anyone's. I think there are plenty of explanations for a Mexican flag on the back of this vehicle. Among them:
(1) This vehicle is entirely unrelated to the sniping.
(2) This was some sort of set up by the sniper(s).
(3) The vehicle had the sticker when the current driver(s) first got it.
(4) The driver(s) put it on, wanting people to think he was Mexican . . . as Mexicans probably attract less attention and are less threatenting than Arabs. I don't recall seeing many Mexican stickers in NOVA, but in some places they are fairly typical - as many here have pointed out.
I certainly don't know the answer, but given the plausible explanations, I don't think the sticker tells us too much. I certainly don't, however, think the sticker should just be taken at face value.