DIAGNOSIS: The thermostat has lost its RF connection to that specific smart sensor. This is usually due to low battery, distance/obstructions, or RF interference.
FIX: Replace the sensor's battery immediately. Use a fresh, high-quality CR2032 lithium battery. After replacing, bring the sensor within 10 feet of the thermostat. The thermostat should automatically rediscover it within a minute. You'll see a notification.
IF THAT DIDN'T WORK: Perform a manual sensor reset. Remove the battery from the sensor. On the thermostat, go to Settings > Sensors, select the problematic sensor, and choose 'Remove Sensor.' Wait 30 seconds, reinsert the battery, then on the thermostat go to Settings > Sensors > Add Sensor.
HEADS UP: Sensors have a limited range (about 45 feet unobstructed). Walls, especially with metal plumbing or foil-backed insulation, can block the signal. Avoid placing sensors near large metal objects, microwaves, or cordless phone bases.