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)
https://nestservices.google.com/partnerconnections/project-id/auth?
redirect_uri=https://www.google.com&
access_type=offline&
prompt=consent&
client_id=oauth2-client-id&
response_type=code&
scope=https://www.googleapis.com/auth/sdm.service
And went back through the process. My thermostats appeared Grey then, unchecked. So I re-authorized there as well.