I was wondering the same thing... wouldn't a cell phone continue to "check-in" with the nearest cell tower?
A cell phone, when on, will always "check-in" with the nearest cell tower. That doesn't mean that it will necessarily do anything though because it is not assigned a phone number per se. I never tried with a newer phone, but with my old phone from years ago I tried once to make a call with no service assigned to that phone and I got some kind of message telling me that I could make a call with a credit card. I had heard too that there are places that will take old cell phones and give them to battered women for use in making emergency calls. So I think you can still call 911 at no charge.
All that aside, the bluetooth end of things is the only thing we are using when looking at occupancy sensing. The phone side of it would not be used in any way.
Another thing I thought may work for sensing the car and would still be of practical use are different car bluetooth devices for hands free phone use. A quick google search got me to this site
http://www.driveblue.com/ which has a number of bluetooth hands free devices available. I am guessing that BlueWatch would see the bluetooth signal from these devices much like it would see your phone. The only issue I can see with something like this is it may be a leach on your car battery if powered at all times, but shouldn't suck much more power than an old cell phone on it's charger.
Dan B.