"legally tied to someone": Dammit, I never rented that driverless car. Yes, I know that it was my credit card and I hadn't reported it stolen, but it wasn't me!
"has not been reported stolen": paid for rentals aren't generally reported as stolen. If you are a business with a driverless car and a wide region of operation, it could easily take longer than a simple "hijacked for crime" to discover and report.
"How many legal trips match that?": who knows. You're speculating, I'm speculating. Unless you have data to show it is significantly anomolous, it is irrelevant. But you *are* arguing for more government surveillance. "Hello citizen, I see that you have been on the road for more than two hours without filing a travel plan with Department of Homeland Security."
"legally, search": random stops? that would likely have to be settled. But there's *always* cause for pulling a vehicle over. And without a human to contest the search...
"It is not enough to obey the laws": true, but the "greater effort" is usually required to escape when one is already labeled. Local police have you fingered as a "troublemaker" you'll discover just how much they can get away with and no lawyer will take your case. But when discussing traffic -- if the vehicle is in proper working order (e.g., no headlights/brakelights out, etc.) and is being used properly (e.g., no traffic violation) unless there is something else to draw attention to the vehicle ("hey, Mark, isn't that the deviant druggie we busted up last night?") they are going to ignore it. Cops don't just go pulling over and searching vehicles on a random basis. (Well, infrequently, at any rate -- they just don't have the time to harrass that many people.)
(Please, don't take this as an anti-LE post. But just like there are good cops, there are bad cops. And if you have the misfortune of getting labeled by local LE it can be tough. And good cops don't go randomly pulling over vehicles so in your scenario we are talking about the less well behaved ones. You bring up the whole in a category they aren't interested in.)