Ah, thanks. As soon as I knew what room to look in I spotted it, but I'm not sure I would have seen it without knowing I needed to look more carefully. It was a little... obscured.
Further hint (bordering on, or crossing into full spoiler):
Assuming you are missing the one that I was - (and it sounds like it) the one hidden off the mimic room - there's two adjacent rooms with openings in the ceiling. One of those openings is hard to see because it's actually a ladder, so due to certain UI choices it kind of blends in with the ceiling. You need to reach that, using the blocks in the room and climb, not just jump up.