We have all worked accidents where, by the time we get there at least, there no physical evidence remains. That is especially true in those instances when the Evidence Eradication Team has hosed off the road. Or when a well-meaning wrecker driver sweeps everything away. It is very subjective and depends a lot on a person's vantage point. There's always that phantom car that only one witness out of five happened to notice.
Some people rationalize.
If they see one thing happen, then something else just had to have happened to cause it. When you hear something often enough, you tend to believe it. Rumors abound after many accidents. Workers at a factory, who hear that one of their co-workers was involved in a car crash, circulate all manner of tales. Some stories are compilations based on several different versions.