DIAGNOSIS The Home/Away Assist feature is using phone location inaccurately, or the thermostat's built-in motion sensor has not detected movement for 2+ hours.
FIX Open the Nest app. Go to Settings > Home/Away Assist. Review which devices are used to set your status. Disable 'Use phone location' if it's unreliable. Ensure your phone's location permissions for the Nest app are set to 'Always'.
IF THAT DIDN'T WORK Adjust the motion sensor sensitivity. On the thermostat, go to Settings > Auto-Schedule > Activity Sensing. Set it to 'High Sensitivity'. Ensure the Nest is not installed in a low-traffic area like a hallway.
HEADS UP If you disable both phone location and activity sensing, you must manually set the thermostat to 'Home' or 'Away' on the device or app. There will be no automatic switching.