Ah but you can't rely on which car gets to where. So the best you can hope for is a to B. Which means both ends are recorded. And likely waymo will look for baggage only trips and report them. Those are trivial to detect with pressure sensors.
It’s just as likely, perhaps even more so, that they’ll eventually embrace and advertise baggage-only trips. Because money. You can use services today (similar to food delivery) to have a random person send a random package, it’s perfectly plausible that’ll eventually use the same automated systems.