It was actually a ‘plugged door’. Some variants of the airframe have an exit door there. This variant doesn’t so that hole is plugged up with a section of ‘normal’ looking fuselage and it has a window in it. It is like the blank spaces for buttons in your car when you don’t have those options installed. So when the whole plug blew out it left an opening exactly the shape of a door.
Lots of planes have plugs like this. Also FYI the plane was two months old.