The thing that is throwing me off, is the lack of phone pings after the garage. If she willingly left with the "walker guy" to go for a coffee/smoke a joint or something, I would expect to see that her phone pinged after exiting. She would have left her phone on, because she was expecting a message from her bf.
If the garage ping really was the last time her phone was in operation, then I guess it's possible that he lured her into his vehicle and attacked her straight away. It's difficult to imagine how he would disable her and grab her phone then drive off without Christina yelling or banging on the window. Is it likely that he had a gun in his car with which to threaten her with? I don't know how someone could drive and point a gun at someone, but just throwing this out as another theory.