Here is my understanding and / or speculation:
I believe she was to have gotten the car from her mom's house on Wednesday night, and that she would've driven herself to work in the morning. I think she might have even generally done most of her morning getting ready routine at her home and not at the bf's house. It's been unclear whether she spent a lot of *nights* at the boyfriend's place or just spent a lot of *time* at his place but my guess is that most of her stuff was at her own home and that she didn't necessarily take a suitcase over there to dogsit. When our son was in college and home for the summer, he did some house sitting but would always come on home in the mornings to shower and change and grab breakfast before heading off to work.
So basically, I think the brother was surprised to see the car they shared was still at home when he woke up in the morning. Knowing she'd have to hoof it home to get ready before jumping in the car and driving off to work, and that it looked like she was maybe running late, he texted her at 7:30 and asked if she needed a ride home (as in he was offering to drive over and pick her up so she wouldn't have to walk / run the mile home).
If I've put those pieces together correctly, I kind of suspect that she returned to the bf's after her run, showered (maybe) and started doing homework, and then someone stopped by the house and visiting with that encounter prevented her from walking back to her own home to get the car at a reasonable hour. I wonder if that person then offered to run her home so she could get the car, and she left willingly with them taking her phone but nothing else.
I know her wallet was at the bf's house. What about her car keys? Do we know if they were there, too? Or do we know if there was another set that would've been in the car? I realize leaving her wallet behind means she didn't have her license on her, but it was just a mile's drive and it's not uncommon for small towns to not have their own police force or for the police to be off duty except for emergencies late at night, so there'd be no danger of being stopped.