I agree that several theories are still viable, although IMO some of those scenarios are extremely unlikely.
A stalker for example. There was too much random movement during Prisma's last few hours (e.g. turning around on the highway to go to her ex's building). The police said that nobody was following her after she left the bar where she spent 3 hours (there were cameras in the parking lot). Also, a stalker would have been seen on camera following her into the ex's building.
I would also discount people living far away (including her son's father). How could they have known she would be in the Olympus building at a certain time? They could not have arrived there on a short notice.
Other possibilities are a random predator or someone other than her ex that she knew in the building (with our without the ex's help). I presume her ex's alibi for the evening checked out.
What remains is an accident or voluntary departure (more likely than not with someone's help). In the first case she would have been found by now. In the second there would have been record of her calling/messaging someone to pick her up. If she left the building on her own (e.g. through one of the side exits) perhaps she could have hidden temporarily in one of the unoccupied buildings in the area. That is, if hiding from police due to the road rage incident was a motive. But eventually she would need to contact someone for help.
We don't know when her phone went dead. If it was at 6:02 (only about 12 minutes after her arrival) when the calls stopped, it could have been due to a dead battery, someone turning it off or damage/destruction. If it was later, it suggests her incapacitation after 6:02 or engagement in something else.