When Victor unlocks the door to let in Margaret, the canvas backdrop moves in the breeze and you can see the edge of a light and the leg of the tripod it stands on.
When an elderly lady is accidentally left in the Meldrews' bed, Patrick watches the entire fallout through the window and relays it to Pippa in detail. As the Meldrews live next door, the only way Patrick could do this is if he could see around corners.