Passive Keyless Entry with BLE.

So I've found other topics that cover the subject of PKE in cars but I had a different approach in mind.

The problem so far appears to be the fob, and killing the cars battery.

I thought of using bluetooth, and when the phone comes in range and latches onto the arduino, the car unlocks and ignition is activated. But I don't want to preclude myself from connecting the phone to the radio via bluetooth.

Couldn't a cheap BLE tracker device (the stuff to find your keys etc) be used as a keyfob, and when the Arduino detects it in range, it unlocks etc? The arduino not broadcasting and only looking for the fob seems like it would also be fairly secure especially in cars where no one expects PKE to be. And it could be named "Kenwood XX-BTU" or something just to deter anyone that detects it.

For the Arduino killing the car battery problem, couldn't Arduino be hooked up to a large power bank, which in turn is only charged when the car is on, running on its own power when the car is off?

Then, if the battery actually dies, the car battery will still be fine, and the user can get into the car since the bluetooth tracker is attached to the car keys anyway.