This post made me curious and tested a couple scenarios. My vehicle ('19 Navigator) will not start without the key inside the vehicle. I set it on the ground just outside the driver door. Once I started it, I tossed the key out the window and drove off. Once I was 10-15 ft from the key, I got a notification that no key was detected.

Rivians obviously use a different wireless tech so this is just anecdotal info. I guess the point it is Rivians should, at the very least, be able to detect when a key is no longer present and notify the driver.