How did LE learn that ML left airport by Lyft or went to the park ?
After ML was reported missing, LE knew ML's last phone contact w to parents to say her flight landed at SLC, so LE could use that time to start checking airport security vids.
Likely that, in reviewing vid, LE 'followed' ML past the luggage claim, outside to taxi/Uber/Lyft stand/area, and identified Lyft vehicle she got in.
Assuming LE got plate info, they could have contacted Lyft driver directly & immediately. Not saying this is how it happened, just a possibility.
IOW, at that early point, no need for subpoenas for e-docs to Lyft corporate HQ, or ML phone record, credit card data, and get investigation rolling. JMO.