Nobody seems to get the question. I have not muted a category, I’ve muted several threads out of the Dustin Fletcher Hangar, so the above (sorry @barnz) is not the answer.
Never mind, I’ve solved it myself. I find that the Search function turns up results for all tracking levels, so I can use that to locate my muted thread and from then on the process is trivial.
Of course, this works only if I can remember something about what the muted thread was called. But it’s better than nothing, anyway.