There are no ideal crimes, for this you need to be committed by a genius, having considered all the options. But even in this case, a brilliant detective can be found, who will find a clue. You can argue that not all crimes are solved, but there are a lot of objective and subjective reasons for this, including negligence of detectives or unwillingness to investigate, as well as banal non-professionalism. The hero of the Stolen Evidence story - Roger Stevens, is just a real professional. He has vast experience working in law enforcement agencies, and has hundreds of open cases behind him. But even he has a couple of hangers that keep him awake at night. One of these cases may advance in the investigation, because a crime has recently been committed, very similar to the previously unsolved. Collect evidence and bring the criminal to justice.
