To help you better understand what has been going on backstage it helps to see the layout, here in a bird/s eye view, Stage left is where we find the light control panel, along the left hand wall. It’s green. 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 hung out in this area when they weren’t on stage. The two doors you can make out in the set are used for most of the entrances and exits. It looks quite likely that Susan and Emily were here through the entire act. Lots of people saw them and it was Emily who found Rex dead in the prop box. You probably have noticed a lot of empty space backstage. If the play required a scene change, the wings would contain scenery to be swapped for what is in center stage during an intermission. So with multiple scenes, backstage could look pretty crowded.