Admittedly, he didn't know all that much more about police work than what he saw on TV, but it struck him as perfectly logical that the police would focus on what they believed were likely clues, and ignore the rest. He also knew that most scientists would take the opposite tack, documenting everything and figuring out later what was useless. On top of that, he was sure that to them, a success was when somebody went to jail. For a scientist, even an experiment which failed was useful, because it made the next experiment better.