I see the CT-100 has a ‘change filter’ light on the lower left side. There’s also 3 dots in a column above that look like they light up. I’ve checked the installation and user manuals and can’t find any reference to these.
Here is a direct response from Radio Thermostat regarding those indicators. I have never actually seen one light up myself.
Here are the reasons: (Yellow Light Events)
When the power is restored after an outage and the thermostat detects battery power only.
When you push the reset button and the thermostat detects battery power only.
When the thermostat detects that the C wire power was removed
The Yellow blinking light will turn off:
When the thermostat detect C wire power
Forty minutes after a yellow light event