I think if Hannah's blood was found in JM's car or apartment, he would be under arrest for suspicion of causing her harm, or murder. In a case without a body, often blood/signs of a crime scene is the best they can do. Especially if he denies having her in his car or home, or denies any sexual contact which could result in blood. This is why they want a statement, Imo.
But if he never took her to his home (which I doubt he would, not living alone) and nothing more than a hair or two show up in his car, if that, where does LE go from there? Even if he lies about having her in his car, that is at best obstruction, Imo. I think they need either his car or home to clearly reflect a crime
Scene, or to find Hannah very quickly and hope evidence is with her, for this case to go anywhere.
As Eileen H. posted earlier...many of the "major" cases in recent times remain unsolved or had acquittals, from OJ and Casey Anthony, to Jon Benet, Natalee Holloway, Madeline M. Etc
Jmo