Currently, this is what I think happened. MY OPINION ONLY!
Kenzie landed in SLC and texted her parents to let them know she'd arrived. She hired a Lyft driver from the app on her phone to take her home and waited at the airport. The Lyft driver picked her up at the airport and saw his passenger was a beautiful lady, all alone in the middle of the night. Instead of taking her directly to her house, he either overrode her destination instructions or said he needed to make a quick detour. The Lyft driver pulled into the park and made a pass at Kenzie. She rejected his pass and he subdued her, possibly ending her life. The driver placed Kenzie in his trunk and made a few more pickups afterwards for an alibi. Most likely any future riders wouldn't need to put anything in his trunk unless they were going to or coming from the airport, so they wouldn't encounter her body. After his shift ended, he did something with Kenzie and her luggage.
The police aren't pointing to the Lyft driver as a POI yet because they don't want to scare him off while they get the necessary warrants and fact check his story.
I hope I'm wrong and Kenzie is alive somewhere just hiding out.