That's what I found:
My XM radio has been cancelled for about 2 months now and it is still working but we are not getting charged? Whats up with that? Maybe free service for good?? Has this happen to anyone else? :dunno:
forums.beyond.ca
Back in the day, when you activated Sirius/XM, it sends an activation signal down when your receiver is on which activates it for good. When you first deactivate, Sirius/XM will send a deactivation signal down every 5-10 mins, after a while, every hour, then later on, once or twice a month. If your receiver is on when it gets the deactivation signal, it'll get deactivated.
From what I know, they've changed the activation system slightly to prevent people from leaving their recievers off for a long time and getting lifetime activations. When you activate it now, it sends an activation signal down that's good for several months. Then it routinely sends an update (daily/weekly depending on how close to expiry) to keep the subscription active for another few months. So even if you turn off your receiver for a long time to avoid the deactivation signal, it'll still expire.