Why does my Gas Boiler keep firing up?

My heating and water is controlled by a timer on the wall, both have been switched to 'off' since last night, how ever the boiler keeps firing up every so often, and the tank in the airing cupboard is really hot and making a rumbling sound like it does when i have the water or heating switched on. Its just been serviced so im at a loss why its doing this? The airing cupboard is in my bedroom and its been hot like this since yesterday making the room too hot, pretty noisy too when trying to sleep!