Once she got there and realized he wasn't there, did she pass out?
Could she have still been in the garage, waiting for him to arrive?
When did her ex return to the apartment complex?
One of the last people she called stated that she did not know the number of the ex's apartment, so she may not have realized he wasn't there. The ex wouldn't respond to her multiple calls made from the building, although he called her earlier that afternoon. IMO it isn't impossible that he directed her to the building, although the road rage event was a disruption that initially saw her speeding away from the area.
She was last seen on video while rushing through the garage on one of the upper floors, though not the top floor. I'm not sure if we can interpret it as running from something, perhaps the road rage incident, or frantically looking for the ex. The latter would likely take place in the residential part of the building where there were no cameras.
That evening a witness saw her on the top floor crying next to a car with an open trunk. She declined a request for help. We don't know when the sighting happened in relation to her calls. Her phone went silent 10 minutes after her arrival. Her last caller indicated that she was in a confused state, not knowing her location.
The ex allegedly returned home at around 5 am.
IMO: If she were able, she would continue using her phone. Had the phone's battery died ten minutes in, the authorities would have known. The least probable (though not impossible) scenario is that she shut down the phone and walked out, abandoning the jeep (and her life, including her young son). The next, she is still stuck somewhere in the building where she climbed or fell. In that scenario, her phone (unless crushed) would keep pinging for a while. She could have been hit by a car in the garage and taken out. An overdose is also possible, although she would have been found and the phone pings would give up her location. The last caller did say that she was unable to turn on location on her phone due to her confused state. A stranger taking advantage of her vulnerable state is another viable option. A wine tasting even took place in the building that evening, so perhaps people not living there attended. Finally, her ex enlisting someone else to kidnap and/or harm her. But how would the ex know she was there?
For some reason she was in a rush to see her ex. If the motive wasn't running from the police, she would have got there, see that he wasn't home and left in the Jeep. Was it a relationship drama? Drugs? Multiple possible reasons for her to act irrationally.