Fearful that a simple software issue could do exactly this, I have adopted an approach to using Waymo with luggage:

1. Get out. 2. Leave the door open. 3. Open the trunk. 4. Get stuff. 5. Close the trunk. 6. Close the door that I left open.

I've had enough stupid stuff happen in a Waymo. I'm not going to leave it to faith that it won't drive off with my laptop, etc.