Bypassing the walker, as for the current moment he seems to be a red herring, I like Tatum's theory that Christina made it to her car and actually searched her car for "missing keys" indeed the keys were missing and not just a desperate ploy to elicit a response from bf.
Regarding possible witnesses to any funny business in the garage, Duncan Donuts delivery person, newspaper delivery person, etc.
Christina's mother said, "“I feel like she was intercepted,” says McElroy, who believes that the key to solving her daughter’s disappearance lies in the vehicles that exited the garage shortly after she entered.?
http://dfw.cbslocal.com/2014/09/05/plano-search-continues-for-missing-23-year-old/
Plano PD said, "Plano police are now convinced that Morris is no longer in the shopping center area and are considering bringing in outside search and rescue teams.
They’re asking anyone that may have been exiting or entering the parking garage last Saturday at about 4 a.m. to contact them. Someone, they say, may have seen something important without realizing it."