Here is a theory. Based on
http://www.uptowndallasgirl.com/dallas-news/missing-woman-christina-morris-updates-and-how-to-help/ (which still works for me btw) being trueish.
Christina is at the party and gets annoyed with her bf.
Uses her dog as a reason to go home and sort things out.
Walking man hears about her leaving and knows they're parked close, takes keys, says he's leaving too.
Christina and walking man chat and walk to garage.
She feels around in her purse and can't find keys. Messages bf asking him to leave the door open.
Walking guy offers her a lift back to the party to find keys.
She goes with him feeling safe, as they'd just walked half a mile together anyways.
Christina is now disappeared.