Open the red door nearest the red key (you can see it from the key location) and deal with the inhabitants. Right inside the door, on the left, a secret door. Just tap the wall.

Turn left right in front of the yellow door. Tap the green face at the end of the blood trench, it's a door. Inside, on the left, one of the walls don't quite touch the floor. Tap and enter; here is your secret.