NYP have access to the Limes CCTV footage so I would have expected the could trace the vehicles down Heworth Road and possibly my to cost cutter?
I would expect they would have an indication of vehicle movements to some degree which were supported by the bus CCTV.
Edit: just to add I think Wednesday evening is more probable due to the phone activity stopping. NYP can again compare activity from other evenings before she had an early start. I would hope this would form some form of routine but made more difficult as she appear to like a drink and to socialise which undoubtedly complicated the whole investigation