It's not a trivial thing to drive it somewhere, park it, and make it back. The logistics of it require some planning.
Somebody that parked it and doesn't want to get involved, possibly an illegal? Even describing how they came about to be approached to park the car, where they got, a description of who they talked to, would of course be immensely helpful to investigators.
At this point I can't see any benefit in not considering such a person at the least an accessory to murder. Calling the police and supplying that information would of course clear them of that charge, assuming they didn't know the person who paid them to park it.
Of course if they know the person then they are committing a crime by not calling police and reporting the person they know had Jennifer's car.
Best to call the police if you know anything about that car being parked at the Huntington that day if you're innocent of any serious crime.
rd