I really don't think it was Sierra herself who threw/placed the bag where it was found.
IMO, things happened in this order:
1. Shortly after the abduction, the phone was tossed from the car (either by Sierra herself, in hopes it may help in tracking/finding her, by leaving it as a clue, or by the perpetrator, so that it may NOT aid in tracking/finding her by pings.
2. Sierra is taken elsewhere nearby, and either left at that location, or disposed of elsewhere.
3. All items that belonged to Sierra, including the clothing she had been wearing, are put in the bag (folded in order to fit inside) and the bag tossed.
I believe the both Sierra and the perp can be found nearby; it would only make sense to dispose of her body, if she is no longer with us, and her belongings in as short a time as possible, and if the perpetrator is someone who works/lives very close to Sierra(as I believe he does), it would be far less risky to get rid of any evidence of her quickly, nearby, than to drive around with it in his possession, especially when anyone and everyone is out searching for signs of her.