Conveniently enough, they sell a device to help with that. "In addition, the Nest Protect activity sensors improve the Auto-Away feature of your Nest Thermostat." (Nest Protect is their smoke detector.)
Yes. I'm not a huge fan of the product in general or the financial deal BUT I will say this aspect of the product is genius in that you want an auxiliary sensor aka the smoke detector right where people spend most of their time, bedrooms and offices and such. There are serious problems with the overall situation, but no sarcasm, the specific situation of this product pairing is a work of genius. I don't think that one idea makes up for the other problems, but credit it due them for this.
"Activity sensors
Nest’s activity sensors have a 150° wide-angle view. That range enables Nest to activate Auto-Away in 90% of homes."
Don't know if its just a 0-D light level sensor or a PIR motion detector or a camera. But it does have a 150 degree FoV which vaguely implied optics.
It has certain architectural implications, my thermostat is on a lightly traveled inside hallway between sleeping and living quarters, so I'll never be able to use a nest; it would consider the house unoccupied pretty much all the time other than bedtime and wakeup time. I guess there are people with thermostats installed in their kitchen and/or living room and/or home office?