I don't recall he looked into it any further. It turns out that Nest Thermostats include both proximity and motion sensors: Sensors in Google Nest devices.

I doubt one in 100 people with these devices realize that fact, and far fewer than that know it's all "phoned home". I suspect it's a bare handful are consciously aware that: "Google knows if you're home, and is watching and listening to you right now." Even if you don't consider it dangerous in any way, it's creepy as hell.