The driver and presumably owner of the vehicle wasn’t on probation.
It was my understanding under California law that:
California says:
If you’re on probation or parole you essentially waive your 4th amendment rights to unreasonable search and seizure. You can be detained and searched with or without reasonable suspicion at any time.
If you’re inside of a motor vehicle, the passenger area of that motor vehicle can be searched pursuant to your probation terms. It’s reasonable to search anywhere that you could reasonably access, it’s reasonable for someone in the passenger area of a vehicle to be able to access anywhere in that area.
If it’s your vehicle, the engine compartment and trunk are fair game also.
No warrants or additional probable cause are needed.