There was a full set of clothing and underwear (presumably sans socks & shoes) in her bag. There may have been a book or two, and apparently there was no makeup.
The early MSM reports said the bag was "secreted" (i.e. "hidden" as if the hiding was an intentional act, not just "concealed from view") which I believe to be inaccurate, but which misimpression has not (to my knowledge) been clarified.
The clothing in the bag was clothing mom knew she had. As I have said before, I think mom might NOT have approved of what she had on for school/skipping school, and she planned to change before getting home from wherever. The lack of makeup in her bag suggests she was already wearing all she needed, and that she was planning on removing or reducing it before coming home. I think the photo taken in appropriate clothing and makeup that morning was part of the deception.
HOWEVER, I do not think she intended to be gone longer than a day when she packed this bag. I also think it was tossed, or placed where it was found later on, but not by Sierra. Had Sierra hidden a bag, it would have been packed full of clothes, makeup, and mementos, and stashed PRIOR to her disappearance so she could leave the house with her "regular stuff," retrieve the bag she planned to take with her, and go.
It is not impossible to believe that she DID have such a bag, due to the lack of cosmetic items or even a brush in the JC bag, unless all of these items are accounted for in the home. She could have taken clothing for a week and if she is like most teens, her mom would not be able to tell what (if anything) she had with her.
Meanwhile, people would be looking for her in the one outfit that her mom COULD verify was missing, namely what she had on in the photo. The clothes were gone from the house, so her mom would logically assume she was in them when they walked out the door. She indicated a desire to highlight her hair... maybe she would change it completely to throw people off if she HAD planned to run off.
I still don't know what to believe.