I had a bike stolen out of the back of my car in the driveway one night. The car had an alarm, but the alarm only sounded if the door was opened. The thief broke the rear window and pulled the bike through without opening a door. If I leave a bike in the car for any length of time now, I run a cable lock to one of the interior securing points in my hatch. Still not a guarantee, but to steal the bike, a thief would have to crawl inside and cut the lock. Not so easy a smash and grab.
One thing I do which will likely never help, but... I put my name and contact info on a sheet of paper, remove a grip, and roll the paper up, and hide it in the handlebar. Should there ever be a dispute whether a bike belongs to me or not, the proof will be inside the bar.
I'll also want to register my bikes through the Garage 529 app.