I don't know if there's truth in what Elaine has said or not, but I do know that in many of these cases the best way to find the missing person is to have the perp lead law enforcement to him or her. It's too easy to hide a body.
Often I don't think you can separate finding the perp from finding the missing person.
Again, I don't know if Elaine is speaking the truth and if Mark had anything to do with Dylan's disappearance, but if one of my loved ones went missing and I thought I knew who was responsible - I'd be pointing the finger and saying very loudly, "ask that person, he knows what happened".