A child safety gate won't stop a nearly four-year-old Sadie, I'm surprised you even suggested such a preposterous notion. They simply climb over them. In any event it has never been proven that any of the gates were closed.
Steps are usually about 7" high, so add that height to the height of the gate and that would be a daunting drop for Madeleine. The steps look concrete with rough brick like tiles on top ... and if Madeleine did try to climb over then she would have left finger prints all over the gate and fibres and probably have had a nasty fall at the bottom of the gate, leaving skin scrapings and ?blood.
Finger prints, fibres nor blood/skin were not picked up by the PJ, nor the dogs.
You do realise, Angelo, that not only was the formidable gates height +step height involved, but she also had to land upright on a narrow step.
But IF, and it is a big IF, she managed all this + the patio door, how come she didn't Yell her parents? No fingerprints there either.
The first thing that she would do when on the patio would be to hear and see her friends and parents just 50 metres away.
Then, how come she had the presence of mind to close the patio door behind her and also the garden gate? She was 3, nearly 4, not an older child who might have done some of these things.
Soz, Angelo, but she didn't go out that way either. IMO