I think I've finally figured this issue out. My thermostat will randomly change throughout the week, from 72 or 73 where I set it before bed, only to wake up to a humid, sweaty-ass 77. I thought I had done this 2 years ago when my former roommate complained about it changing and I went deep in to settings. Apparently I didn't go in to the Nest app, just tried changing settings in the Google Home app. Open the Nest app, tap on your thermostat, tap settings at the bottom, you should see a calendar with a bunch of random temperatures set throughout the week (whether it came from the factory that way or you set them when you got the thermostat and forgot) Tap on each day, then tap REMOVE at bottom right, then tap the temperature you want to remove. You have to repeat this for each temp, tapping REMOVE then tapping the temp to remove it. I removed them all and so far it seems to be staying on the temperature I set it to without changing. Remove them all or set your own at the temp you want. But I think this is the solution to randomly changing from where we set it to 77 or 90 or 60. Attached screenshots are of mine after I removed all the pre-set temps. Mine was set with pre-set temps all over the place throughout the week. Hope that helps!
Glad you found the schedule in your Nest! Thanks for posting the explanation! Yes, that's how it works -- the Nest is, at heart, a programmable thermostat that runs on a schedule. You can perform temporary overrides or brave the thicket of fancier features, but the main way to control the Nest is to set up the schedule to your liking.