Police should bring in an expert such as myself only after they've worked their current information. When Susan disappeared the police should have pinged her cell phone to establish if her phone was working and her last known location. The police should also polygraph Josh's father as well as Susan's husband Josh.
Next let me bring in a forensic detection dog train to detect minute amounts of human live and cadaver scent. We can then show the police if there is any "Death, or Stress alerts" inside Josh's or his father's vehicles. Or on any of their tools such as a shovel.
Next bring me out to the "Caves" the police are interested in. This will show if Josh, Susan, or Josh's father was in any of the caves and we might be able to detect if she's still there dead or left on her own power.
www.k9sardog.com author of "Foresnic Scent Evidence" and "Search and Rescue" by Harry Oakes