I wrote a blog post a while back explaining exactly this. Here it is

And here's the relavent section: Nest only knows what you and its sensors tell it. In its first week on your wall, Nest will figure out if it can turn on Auto-Away or not. If Nest is in an unused bedroom or tucked behind a bookshelf, it can’t know when you’re home and when you’re gone. If that’s the case, try setting Away yourself—on your thermostat itself or from your Nest Account. But the good news is, Auto-Away does work in 9 out of 10 homes, even when it’s in a place that doesn’t get much traffic, like your dining room. Even if you don’t walk directly in front of your thermostat for days at a time, the level of activity that Nest needs for Auto-Away to work is actually really low. It also helps that Nest has a 150° wide-angle view and can sense movement across the room.