The reason why Shannon's mum is not banging the door down at the Police Station to get her kid back is because she wants the Police to find out from Shannon, exactly what has been going on.
What made her want to run away before she went missing. Was the Step Father abusing her? If she was my daughter, I'd want the Police to get tot he bottom of it, and if that meant a few more days without her at home, so be it.
Imagine if she is being abused by him at home, and the Police just sent her back there.. then what? We'd all be horrified wouldn't we, that the Police didn't investigate it and something bad happened to her, after her ordeal.
Lateral thinking. It goes a long way.