I might be mistaken, but if I remember correctly, my Audi only beeped if you close the doors and they are locked, essentially locking the car with the keys inside. That’s a near impossibility in a Tesla as you would have to press lock on the touch screen since there is no physical lock switch.
My guess is there is no beep on Tesla’s on walk away is there is no way to tell you have actually walked away if the phone is still in the car. Yes it can tell you are not sitting in the car, but what if the driver gets out of the car and walks to the trunk, or walks around to the passenger side to let your girlfriend or mom in the car? Could it then be timer based? If timer, how long would it have to be without beeping unnecessarily and becoming annoying in those situations? Might someone get far enough away where they don’t hear it?
Might be a simple feature to implement to beep when you leave the car and lock with the keycard and the phone key is still inside...but honestly how often would someone have both setup and use them like that?