You'll obtain this without really trying. Items are marked by evidence markers (yellow bifold signs) and should there be one without an accompanying article (an evidence item or a corpse), the police will get closer to investigate. Furthermore, as you're transporting corpses to body drops like your vehicle or stage-specific locations, you might end up having to drop the body to hide. As a cop notices a bag in their line of sight, they'll get closer temporarily to check it out and see if there is anything else interesting nearby.