I wonder do the Police think she somehow ended up in Oak road Playing Fields by eliminating certain routes from CCTV footage. I can imagine there is CCTV along most of the main roads at some point, so it would make sense to eliminate each road as a possible route. Although I have little doubt PW has sexually assaulted her while he was "helping" her, following this I still think the most likely explanation is she somehow ended up in the River Hull, which would explain why no body has been found.
I think this is how they have come to that conclusion and why they are focussing on that area so much. They did release a statement a while back saying they have viewed over a 100hours of CCTV footage. So they must have ruled out her walking other routes and that is why she hasn't been seen since a certain time. Or she has been picked up out of view and traveled out of the area via car.