Many boiler installations are fitted with a frost-stat - to stop the pipes freezing up even when the timer says the boiler should be off. Often found in an attached garage - nothing to do with the boiler specifically but part of the installation. A trigger point of +4° C is usual.
It may well be that during the day a) you're not there and will not therefore notice - or b) if you are there you will have turned the heating on yourself - or c) the temperature will be greater than the frost-stat threshold.
Good Luck..., January 2016
