Maybe she went out the front door and it slammed behind her leaving her with no option but to follow the path she was familiar with?
The front door is so easy to open with the lever from the inside, easier to open than the gates on the stairs.
Problem I see is that the door had anti-slam features. You need a key to close it behind you, you can't just push those doors shut.
David Payne confirmed this when he said he rushed over there to check if the front door was open but it wasn't. Could the door go into a half closed position (one that looks closed but not fully latched)?
Kate hadn't noticed the door unlatched. Who would expect someone coming through and closing it to look like Madeleine had been abducted rather than run off? Why is Dianne so concerned about the shutters?
What also concerns me is understanding how David Payne gets into a position to check the front door as he says he did? Did he go in through the patio door and up the hall way or did he go right around to the front door and then found it closed?