If he took Sierra to the spot where the bag was found...she would have been found there too, IMO...no way he would carry her back to his car, etc...
I can't see a female perp or accomplice hiding the bag for later-why hide it at all? Just stick it under the seat of the car or in the trunk, if you want to keep it-no one was "after them".
In fact, the only thing that makes any sense to me is the perp coming back from taking Sierra away and realizing he still has it in the car and just wanting to get it out of sight in a hurry, so he finds a desolate spot and hides it...nothing makes much sense though. I still think he would probably have gone through it, just to see what was in there, once he had time and yet it appears not, unless he re-folded her clothes, or put them in there himself. Or is was someone who knew there wouldn't be anything much in there...
I wish someone would have asked Sierra's mom or LE if she always/often sent photos of herself to her friends in the morning...especially if this was a friend she would be seeing in a few minutes. It is not like the photo displayed her outfit, which I could see a girl doing. It was just a silly face and no message, until a text afterward...
Anyone could send a photo from her phone...and then send a text to make it look like it was Sierra. So that kind of bothers me.
JMO