Stubborn people must install GPS on their cars.

I have the X200 Omni installed on all my cars including the parents. Don't care if they protested or not.

As a backup, I added a Katsana tracker on their cars too. Hidden from plain sight.

Another way is to track their phones via Find My Device (Android or iOS).

The old folks can protest all they want but as long you're not living with them, this is peace of mind.