It's the one room on the first floor (seen in the bottom cluster on the right in the pic). The room to the left contains a snaking corridor of too long to traverse before drowning water, on the other end of which is a button. The room to the bottom contains a ledge that you can't leap up. I was thinking you might have to drop down into the room, but that doesn't seem to work.
This is the first time I've been legitimately stuck in this game (which is lovely by the way), so please help by pointing me in the right direction. TIA!
Ya. My problem was that I somehow spaced out on the dust cloud you can pick up from the top right corner room and ride all the way around to the room I hadn't accessed. But after progressing further through the level you pick up another key that it appears is just an extra.