How does it not trigger the alarm once the car is locked?
And if that exception can be coded into the car, wouldn't that still drain the battery fairly fast?

Also, afaik some cars offer a lot of stats via Bluetooth (in my case various temperature of hvec, exhausts, interior, gears, power, torque, battery status, fuel left in litres, fuel used in trip, etc), so you might not need the OBD port.