I took down a small section of drywall as planned, hoping to be able to do something with a little precision. And I found..... nothing, except a draft coming in through the gap I made in the drywall. So I took down some more drywall, and some more, and some more, and took the trim off the door, and took down some more drywall, and so it went.
Now I have probably about 50 pounds of drywall in a garbage sack, a few featherboards taken off the wall, and still didn't find what I was expecting. I did find a nasty draft coming around the drywal on the perpendicular wall, found a small hole in the inner face of the brickwork that had a draft coming in through it, and found I hadn't fully blocked the nasty hole in the brickwork that someone made to fit an electrical back box. In time I'll fix it with mortar and cement but for now I've fixed it by stuffing fiberglass into the spaces. Hopefully I can stop it getting too cold in there this winter. Since I drained that heating loop there's no source of heat in the room at all, so fixing drafts could be a quick and easy way to not only save some money but also stop it getting quite so cold in there. Last winter I ran an electric bar heater just to keep the chill off, it would be really nice if I didn't need it at all this year.