What exactly is Auto-Away and Nest Sense?

For those unfamiliar, the Nest Learning Thermostat includes a feature called Auto-Away. Auto-away is a mode that the Nest thermostat enters when it detects that your home has become empty.

Auto-Away relies on another Nest feature called Nest Sense. Nest Sense relies on a motion sensor on the front of the thermostat. If the sensor doesn't detect motion after a period of time, Nest Sense considers the house to be empty and will enable Auto-Away. Conversely, if Auto-Away is engaged and the sensor picks up activity, Auto-Away will be turned off and the Nest will resume to usual operation.

Nest Sense has an interesting capability. It will calibrate itself over time. If it enables Auto-Away and then, later realizes through user intervention that it was wrong, it will wait for a longer period of inactivity next time before enabling Auto-Away. Similarly, if it feels that it's generally right, it will decrease the period of inactivity required before enabling Auto-Away. Nest has previously reported that this inactivity timer can range from as short as 15 minutes all the way to 2 hours.

Nest Sense and the Auto-Away feature was failing me at my home.

For one, I work from home some days. In the winter, I frequently would come to realize that the house was getting cold because Auto-Away had activated without me knowing. Second, I am in and out of the house quite a bit, without a set schedule. I didn't like that it would take up to 2 hours for Auto-Away to turn off my furnace each time I left my house. My furnace is underpowered for the size of my house. In the dead of winter, it often works pretty hard to keep the house cold. It's more likely than not, at any given point during the day, that the furnace is running. If I leave, that's potentially 2 extra hours of burning natural gas.

This was frustrating. I had ran out and purchased a $250 thermostat in hopes of reducing my utility bill and also automating the climate control of my home. It seemed like I was close with Nest, but this Nest Sense / motion sensor business just wasn't cutting it. I considered other sensor options:

Turn off the thermostat when I lock the front door: My housemate likes to keep the front door locked when she's home during the day for security, so this isn't an option.

My housemate likes to keep the front door locked when she's home during the day for security, so this isn't an option. Turn off the thermostat when the garage door opens/closes: It'd be difficult to know whether a car is leaving or arriving.

It'd be difficult to know whether a car is leaving or arriving. Tether off of the home/away status of some other smart device at home: Somewhat by choice, I've kept most of my house 'dumb'. No good options here either.

Somewhat by choice, I've kept most of my house 'dumb'. No good options here either. Rely on phone technology to determine if I'm home: Now we're talking!

I came across a discussion thread regarding this issue on the Nest Community site. I learned that many users were having the same problem as me. Nest even has an official page addressing the issue. It turns out I'm not so unique after all! Some solutions were proposed. One member had created a PHP or Python script to continually check for their mobile phone's MAC address on their wireless home router, but as mobile phones often 'sleep' when not in use, it was tough to verify if the phone was reachable or not. Other shoestring solutions were proposed, but generally also lacked reliability or only worked for tracking a single person in the house, not the whole family.