I ran into this situation last year. Filled up tank to the brim; was charged $13.99 for fuel. This led me to read more into the connected car abilities. What's scarier is that Avis is constantly monitoring your GPS location while in an Onstar vehicle. I posted about this incident here:
Avis Chevy Malibu incorrect fuel service charge
I look for Avis to get into a legal debacle at some point in regards to the "not being certified by weights and measures" thing. Sad thing is, they're still better than Hertz lol
TL,DR: I had GPS/"location services" turned off on my phone, yet when I was pulling into the rental car return area, the Avis app gave me a push notification saying "welcome back!". The only way they were able to get my location was through the car's GPS. On this same rental, I was charged $13.99 for fuel even though I had filled the tank to the brim.