It is not uncommon for a boiler installation to incorporate a Frost -Stat - to fire up the boiler when the temperature drops close to freezing - to stop the pipes from freezing up.
It may well be a frost thermostat doing this for you - I think they place them in a cold area - a garage for instance - a frost-stat will ignore the temperature of the house and the programmer and do its own thing.
Consider the outside air temperature when you've noticed this condition...
Good Luck..., February 2016
