The possibility to spy on you exists. Will they spy on you is another question.
I found out that rental car companies will use it to spy on you. I dropped a GM car off after renting a few months back and, because there are few affordable gas stations near the airport I was travelling out of I usually stop a gallon or so worth of gas prior to the airport.
Turned the car in and got my receipt. A week later I get an additional charge for 0.9 gallons of gas, at $8 a gallon, that I believe OnStar reported I was short when I turned the car in.
rental car companies have done this since before OnStar...sometimes they just take your car and see how much more gasoline they can get into the tank and charge accordingly, they could have checked the gauge, they could have checked gallons used and gallons remaining if so equipped...I wonder how you concluded OnStar reported you...seems a bit far afield for me to believe.
I highly doubt that was OnStar's doing. What was the rental company?
What most likely happened is one of the employees took it to their pump to top it off and charged you for it. I've rented plenty of GM vehicles from Hertz, Enterprise, and Avis with OnStar equipped; had to drive a ways from refuel to drop off and I was never ever charged for a drop of gas when required to return "full."
I have to agree with the two responses above. I have to doubt that On* "spied" on you or provided information that was not specifically requested by the company. Reaching that conclusion seems to me to have a little too much paranoia of "Big Brother" mixed in. Not saying it doesn't or can't happen, but quite often the simpler explanation tends to be the most likely.
I have to give you credit, though, for saying that whether they do or not is another question. Yet the very next sentence goes past the suggestion and claims that it has happened. While it may
be true (see my comments above), I'm of the mind that I'd like more proof than just an anecdotal post.
I've taken a few rentals back and the gauge was close enough that no other charges were incurred. Check the contract where they circled or indicated the fuel level when you took the car. It may have been off just enough that an astute employee noticed it, resulting in your bill.