My guess is one or more people at the park, partying throughout the night, saw Cheryl pull up and guessed that she was younger than she is. It's possible that it was a bit of a blitz attack where she was approached on both sides of the vehicle, her phone was taken from her, she was pushed into the passenger seat, and the car was moving.
Let's suppose that this is a random attack near the location where her car was parked. The attacker(s) do not know that Cheryl is meeting a carpool. They assume she's taking public transit.
If her body had not been found, and if her carpool was not expecting her, the situation would have been that she parked near the Park and Ride, and she took public transit to wherever. That is a very good reason for the perp to return her car to the right general area, because, assuming that she was going to take public transit, no one would have any idea where to start looking for her. If her car was not parked where it should be, police would know that she was abducted in the morning when she was parking her car. If her car is where it should be, and the perp assumes she normally took public transit, the first place to look for her would be near her place of work.