This bird's eye view of stage left should help you understand what was going on there during act three when Rex disappeared. Stage left is the location of the light control panel, along the left hand wall. You can see Susan at the panel checking out a new lighting preset. Also below on the left you might make out the stairway leading to rooms below the stage. It is where you go if you need to talk or make a phone call during a performance if you don’t want to go back to the dressing rooms. In the third act when Rex disappeared most of the actors remained in this area when they weren’t on stage. The two scenery doors you can make out in the set are used for most of the entrances and exits. It seems almost certain that Susan and Emily were here through the entire act. They were very busy and lots of people saw saw them.. There is a brick wall at the bottom of the image. On the other side of that wall is the dressing room corridor which runs from the stage entrance to the prop box area. You can actually see the prop box from the stage door. Remember that Ros says she was in the ladies dressing room sewing during all of act three. Did she go down the corridor and meet Rex at the prop box? But if so, Nick must have seen them..