The camera would no longer be recording if it is not tied to a paying Xfinity account. It would be the homeowner's responsibility to remove any old equipment.
The camera is still getting power though if you are seeing the LED's light up. Even when not connected to an account, the camera may switch in to Night Vision mode which lights up two red LED's when it gets dark.
My best advice if you want additional peace of mind is to follow the cord from the camera until you hit a power outlet, and then unplug the camera's Power Adapter. Our cameras do not hardwire in to power, they have a plug just like most electronics - at the very least, there is no reason to be powering the camera if it is not in use.
I hope that helps!