Ever heard of a “Cold Case”.
Detectives refer to a cold case as an old case which was left unsolved. The original investigation stops due to a lack of new evidence or leads and the case is neither open or close, it just goes cold.
Detectives however tend to look into cold cases at a later date. Sometimes cold cases are reopened as a pastime or just to hone skills. In any case it’s a productive way of staying active and results in doing some good not just for the detective but for the community, the victim and the police department.