Without the DLC, the secret room would've been here guaranteed, as it's the only non-room area with three or more rooms around it:
Instead, I lose. I get nothing. Good day, sir. Why is that? What changed in the DLC?
Super-late answer go!
Secret Rooms always spawn adjacent to three different rooms whenever there is such a possibility. Super Secret Rooms try to border only one room, but they are placed first.
The answer is that the Secret Room is either below the treasure room, with a Super Secret Room below that, or to the right of the shop, with the Super Secret Room to the right of it.
And to answer a comment, as I can't comment yet:
Assuming this isn't a bug, you didn't find it.
Sometimes Secret Rooms are attached to curse rooms now, which confounds things a bit.