Not only does it know when you're home but the Nest Learning Thermostat also knows when you're nearby. Here's how it works.
You crank up the heat to 70 and walk away. Nest then immediately returns to 62 degrees.
Thinking there must be something screwy with the algorithm you turn it back up to 70. Nest knows that it's in trouble so it displays a comforting message like 'Heat set until 10pm', waits for you to leave and then sets the temperature back to 62 degrees.
Giving up on the learning part you use the app to manually program it to keep the heat on. Nest now uses its WiFi connection to phone the gas company and disconnect your service.