There's the added complication that the vehicle was in its registered driveway. How is a vehicle "stolen" if it is parked right in the driveway where it belongs? For all police knew at the time, Gabby parked it there then left for somewhere else.
Theft is generally taking property with the intent to not return it. Since the van was parked right where it was registered I think the best they could do is maybe a charge of unauthorized use of a vehicle. Which I believe is a simple misdemeanor.
And LE had much, much bigger fish to fry at that time. jmo
ETA: I was wrong. Unauthorized use of a vehicle is indeed a felony; just not as bad a felony as grand theft auto.
Do we actually know that was its "registered driveway"?