He has been charged with possession of a stolen phone so it is possible he has access to other phones other than his own that police dont know about
I fear that he may have attacked Libby in the park, accidently (probably) killing her, turned off his known phone in the park to give himself an alibi and used a back up phone that the police have not traced so he could travel out of Hull with her body in a different vehicle to his, the last ping traces of his known phone aren’t necessarily his last location with Libby
I am 90% certain he had a spare phone which he has probably dumped somewhere, and access to another vehicle
The park just feels like a red herring to me, I doubt she is buried there or that she was thrown in the river