The padlock will appear on the thermostat whenever a button is pressed but the action is not permitted.
For example, if all functions are locked, pressing Heating (radiator icon) will have no effect and the padlock will flash.
If Restrict Set Points is disabled, the target temperature cannot be adjusted. When the up or down keys are pressed, the padlock will flash instead.
If Restrict Set Points is enabled and you try to increase the temperature above the maximum allowed set point, or decrease it below the minimum allowed set point, the padlock will flash. For example, if the Max Set Point is set to 24°C and you try to increase the temperature beyond 24°C, the padlock will flash.