AR told LE that the uncle had told Brooke they were going to 'party.' I think she was waiting around town for AR and the uncle to return.
I think I agree with you, in that something most likely happened the night before. At least, to my inexperienced eyes, Brooke looked a bit distressed at the convenience store, arms crossed protectively in front of her body. She was way too quick to take off the opposite direction once outside the door as well.
That's the part that has me stumped. Why did the uncle take her into the store and have them each walk out separate ways, not even turning to say good-bye? For the benefit of the camera? For an alibi?
Premeditation of what was to come?
Heck, even AR said he threw out his coffee.
JMHO
fran