Shape Shifter

I do believe our cat Amber is a shape-shifter. We have a door on the family room which closes off the lounge to naughty cats and dogs, you know, the ones who sleep on the couch and scratch tapestry chairs. This door has a large center pane of glass with four long and four small square panes around it. We don't have the outside panes, not from laziness or lack of cash but because the door serves its purpose and the glassless areas allow the breeze to slide through.
Every morning I shoo Daisy through to the family room and carry Amber through. I close the door. I repeat to myself "the door is closed". I go to work. When I arrive home in the evening, sitting on the back of the couch in the front window is, guess who, Amber the cat that's who. How does she get into the lounge??
So, we have a few possibilities
1. Someone unknown sneeks into the house and for a prank lets the cat through - unlikely
b. This rather fat cat is able to pull in her tummy, shrink her head and slip through the (70cm or so above the ground) empty window frame which is no more than 10 cm wide.
I saw a picture of a cat of the same colour stuck trying to get through an outside window, it apparently can't be done.
So, that leaves shape shifter. I will have to borrow a motion detector camera, place it on the fridge, facing down the hall and see how she does it.