I think that the driver of the car was unknown to Sierra, he/she/they? didn't choose to remain in that area. Too risky. Not going to find her near home.
One old monster: If it was a single older man he had no way of knowing at what point in time Sierra would be discovered missing. What if she had planned to meet up with a friend before class that morning? What if there was an emergency and a family member couldn't reach Sierra on her cell? What if her home room or first period class, or even a classmate called her mother to report Sierra was absent? How much would an older man know about the habits of teen? How much would he be willing to risk? He might be gambling against the clock, and desperate to hastily settle his lustful needs. He may have already been on record as an SRO. This one is extremely dangerous as he would not leave a witness behind.
Two against one: Now, if Sierra knew at least one person in that car, she may well have accepted an offer of a ride. In that case, if there was more than one person in that car, she was outnumbered from the start. I'm still thinking that somebody had to keep her hidden from the other commuters, and to render her unable to fight/flee. If on a freeway there would be no need to stop at stoplights, so visibility may not have been a concern. If she had an opportunity would she risk leaping/falling from a moving car to whatever horror she would have to endure from predators?
Friend: Another option, if she knew and had arranged to go with the sole male driver of the car, then there are just too many possible outcomes to ponder.
Classmates: Another iffy if females. There are some really mean girls who have no problem eliminating the competition. Did Sierra steal someone’s guy, even much earlier?
I don't think there is any hope that Sierra will ever greet her family and exchange hugs again. I feel sorry for this teen, and sorrier for her family who most likely will have to live with grief and loss for the rest of their lives.