Not sure exactly how Nest works, but from what I understand the sensors that are there don't necessarily directly turn the thermostat on and off, rather it tells the system when you come and go. The idea is that the house will preheat or precool the house before you walk in so that you come to a house with the perfect temperature. Though thinking about it, leaving the house might directly turn it off.
Either way, I'm not sure if I understand the system correctly, or if there would even be a heter the way I described it to be.