I am sure that her cell was tossed immediately. That leads me to wonder if she was already subdued in some manner, and not visible from the passenger window. Perhaps someone got cuffs on her, or had an accomplice holding her down.
I wouldn't be surprised if she was eventually dumped in a well-hidden very remote location quite far away from her home. The longer she is undiscovered the harder it will be to gather trace evidence, and a river or lake may have been chosen. The car, well that may have been a rental car, already returned if this was all pre-planned. Could be like looking for a needle in a haystack.
There are so many children missing, and I think that when and if found they will not be close to home. The exception being a parent/step-parent who was involved in their own child's "disappearance".
I still go with my original theory that Sierra's bag was not disposed of at the time she was abducted. It was placed there "after" whatever was done to this girl, and probably because it was like a red flag, saying" lookie here". I don't think many murdered children are buried underground anymore, and even predators are getting more savvy about leaving evidence behind. It is my understanding that water washes away surface evidence. OMG I'm too old to be thinking about this gory stuff, but then I think "what if this was my grandchild?" and I get emotionally entangled in another new case.
I wish all children were ear-marked or toe-marked with GPS locators. Privacy issues become superfluous when lives are at stake.
IMHO