When the police are investigating a crime and preparing a case for the CPS not only do they have to prove what has happened, but have to prove what has NOT happened. Robert has accused John of harming Mack, so at the very least they should try to find out where Mack is and if he has come to has or has not come to any harm. Which means they should talk to Charity to find out where he is, when hopefully she would tell them about Mack's message, or to be more correct John's fake message.