OK, DEEP BREATH, people.
Here's some facts:
1. The Google App has to be running before it is listening.
2. In iOS 8, Apple devices will have an option (which you can disable) for Siri to be listening for you to say, "Hey, Siri", at which point it will act just as if you pressed your home button. Further, it only works when plugged in (due to the adverse impact on battery life).
3. Capturing the unique address of your Bluetooth headset won't identify you at all since there is no registry of any sort as to who has what Bluetooth address. Even if the car manufacturer noted the address of a built-in Bluetooth in your new car and tracked who bought it, it obviously wouldn't be of any use once to car was resold. There's zero privacy issues here.