I lived on SI at the time but had not even started kindergarten yet when she went missing. From what I can recall, the theater was in a large shopping center near the mall, but not in the mall or even in the mall parking lot. It would have been quite a walk home from there, so if she did have any money whatsoever, and she got out of the car after an argument, she more than likely would have walked to one of the stores (or even the mall) to use a pay phone, but it sounds like she didn't call her parents - or maybe she tried, there were no answering machines/call waiting/etc then. Or she could have caught a bus anywhere along Richmond Avenue, and it would have taken her close to home.
If she did walk, the majority of her walk would have been on Richmond, which is a busy street, not a likely place for someone to grab her, especially when it was light out.
Maybe not grab her, but thinking maybe she would have accepted a ride from someone, back that it was common to put out your thumb if you needed a ride, just a thought.