Could only be arsed to read the first 8.5 pages, so don't know if it's been settled, but Paul's hidden heat source is probably underfloor heating. The concrete is laid on top of the pipes to add thermal mass, enabling the heat to convect better. Although in some situations the floor can feel warmer this is not necessarily the case. Underfloor heating works best if it is on all the time, because it takes a long time to heat the air space, that said it is still more economical because a lower temperature is required to achieve the same ambient temperature. If it is the case that it is on all the time, running at a low temperature, it would not be instantly noticeable from touch. However, concrete, stone and plaster surfaces are inherantly cold to touch, so if the floor does not at least feel cold, then it is likely that it is underfloor heating.
The only other option that has not been mentioned is skirting heating, but this is usually fairly obvious due to an opening towards the top of the skirting.
Oh, and it's been annoyingly mild here today.