Well some power is used to run the fans which ends up partly turning into kinetic energy of air leaving the case. Sure, that may also manifest itself as heat so

Well some power is used to run the fans which ends up partly turning into kinetic energy of air leaving the case. Sure, that may also manifest itself as heat somewhere down the line, but not in a way that is relevant for this discussion (and indeed you have to draw the line somewhere). It's probably fairly neglible though, probables less than a watt in most systems.