I was using the Smart Device Management API flawlessly for over a month, then this week.. 1 of 2 thermostats disappeared. (A couple questions on StackOverflow saying "There should be two as I'm setting this up."), but I HAD both of them. Just today when I pinged the API, both thermostats are now gone. My 2 cameras are still on the API device list.
So they were there, now not. The only thing that has changed is I upgraded my Nest account to Nest Aware.. but that shouldn't impact the thermostats' availability in the API?
For anyone that runs into this (apparently rare?) problem.. I re-auth'd the Thermostat into my Google account, then went to the Device Access authorization link (first step in OAuth process)
And went back through the process. My thermostats appeared Grey then, unchecked. So I re-authorized there as well.