I don't know how the smart thermostat is supposed to know your behaviour unless you tell it, which means taking it off the default. I had to tell my thermostat explicitly when I wake up.

With room occupancy detection, it should start modeling likely HVAC needs - able to predict accordingly, and adjust power use & timing jitter accordingly.

Ah, that's another level of smart than I was thinking of and requires integrating motion sensors, but I do suppose that would enable it.

CO2 sensors are the most common way I believe.