OK, lets look at this................
Just because you canceled your subscription doesn't mean they cannot and/or are not monitoring your vehicle. The signal is always there. Not sure how they monitor be it cell signal or some other way. Someone has to be footing the bill and I don't think onstar will be doing this without being paid for it.
The next thing is does the vechile transmit all the time or does something have to request the information? If it is transmitting all the time then it would be nothing for someone to monitor it. If a request has to be made then that will cost some one something thus the subscription.
I am not sure how the system works but if you are paranoid then disconnect the feed.
