what are some out of the ordinary heat sources one might have inside ones home?

My ketchen for some reason gets warm for no apparent reason. I have checked the appliances and I can detect a heat source outside normal limits. This heat source seems to pick up late in the day, however this morning after a night of 69 degree weather (outside) the ketchen had enough heat that when I opened up the door to there was enough heat in the ketche to raise a 500 square feet area 8 degrees in 20 minutes. This was also 8 degrees higher than the outside temp. Has anyone ever found some odd heat sources which a person aware of the usual heat sources might miss. I have never run accross such a conundrum.