If I had more information or all the facts, I might have a different view, but I think the security guard scenario makes more sense than someone taking them against their will. If someone forced them to come with them I dont think the guy would have tried to do it in the mall and even the walk from the mall to the car in broad daylight doesnt seem like it would appeal to an abductor. Too much traffic during Xmas , that is if the parking lot was the one on the Seminary drive side.
If they were kept in the basement till late in the evening they might have left a clue in what ever room they were in.
Of course, we don't know if the girls made it back to the car. It might have been that the unsub was waiting for the girls at the car and when they got in, he jumped in and carjacked them and later returned the car to the parking lot - which is why he knew where the car was parked for the letter. In other words, he may have seen the girls go into the store, parked nearby and waited, then started walking past their car as they got in and suddenly jumped in and carjacked them - passerby's might not notice that type of thing. If he had a gun, it would be easy to control the three girls.