We assume Sierra has two chargers
We have assumed Sierra had secret plans to visit dad
We assume she packed clothing for a clandestine visit to her dad's
We assume her dad/sis was going to pick her up from school because they don't like to drive to her house.
We had to assume she packed neatly, contrary to her normal behavior.
.....I think all of these assumptions have to be made to justify a belief that the bag was preplanned by Sierra. In doing this, an awful lot of assumptions were put in place to justify this position.
....too many, IMO.
The last assumption is convincing. I could see her missing her sister, asking her to come get her, even packing a bag in hopes that it could happen. But how she packs a bag wouldn't change from one day to another, I believe. Unless maybe she was in a hurry? IDK.