No, maybe I wasn't clear. Nobody is using the front door (not even Gerry because he couldn't be arsed walking round, hence why he left the patio door open). The door may have been opened by this abductor as a secondary means of escape. Nobody in their right mind would walk out of the front door not knowing what's going on out there, when you have a perfectly reasonable mode of egress through the way you came.
OK I might have it a bit wrong, but I find you are making a couple of assumptions you can't make.
1. That Madeleine was in her bed asleep.
2. That you entering in via the patio door wasn't enough to make Madeleine escape by the front door.
"Thank you. So you have a perfect scenario for your abductor: illumination, an open back door, using the most secluded and gloomy access point with a simply opened front door, nobody home.
Rush in through the patio door, maybe listen for a few seconds, walk straight to the front door (or not), put it on the snip or open it, take the wee an, straight back down through the way you came, in to the car - whoosh! Gone. Takes literally seconds.
So why the window?"
So you have failed to abduct her if she escaped, so why the window? Well here is a possibility, you make it look like the window was the entry point,and you close the front door again, and quietly exit via the patio door closing it all behind you.
Making it look like someone else.