In a contact shooting, pieces of flesh are blown into the gun. Forensics found traces on the patch that matched the DNA of the victim. The case was salvaged and the guilty party was convicted, but not by a ballistic comparison. We readily accept the abilities of the bloodhound to trail and find missing persons, or prison escapees. We now know that the dog's nose differentiates the scent of a specific individual.
Cases mentioned above prove the dog's ability to identify one who left scent at a crime scene.
It's just a matter of having the scent for the dog to follow. Their scent is there. You may have a hard time finding it, but it's there. It is now standard procedure in the Niagara County Sheriff's Department that scent is collected at every major crime scene. It is stored in a scent bank.