I agree with Klos - the sniper does not have a white van, and never has had a white van. It's called the power of suggestion. Another example of power of suggestion comes from witnesses who claim to have heard a gunshot. ("sounded like a tire blowout"/"sounded like a car backfiring") I have heard on TV from ballistics experts from several different channels that the gun the sniper is using is silenced, so no gunshot will be heard, just a 'thud,' which is how a silencer makes a gunshot sound.
But, people are stupid, so what can you say?