I have leaned towards the theory that the intruder had a key to the wooden door and made entry from there
However Horrocks' notion of how entry was made is worth looking at and it is something which the forum has previously discussed.
http://miscarriageofjustice.co/index.php?topic=6209.msg232924#msg232924Horrocks says "These actions could be seen from within the Ocean Club area, as well as from the alleyway that runs between this and the apartment. Due to the height of the wall and foliage on top of it, as well as the area inside being well lit, in contrast to the darkness elsewhere, those dining would have been easily observed whilst anyone in the alleyway could remain unseen. Sunset on the 3rd May 2007 was at 8.25pm, so it would have been getting quite dark by 9pm.
Anyone observing their routine would have known that they had at least 20 minutes between each check. They would have observed the group for a few minutes and then gone to the apartment. At the end of the alleyway they could see that the road was clear, it is then only literally a second for someone to go through the gate and into the garden area, where they would be virtually out of sight. It is then simple to enter the apartment through the patio doors, which although closed, had been left unlocked." End Quote
I don't think it would have been necessary for an intruder to be exposed using the road to the gate ... it would have been easy enough to get across the wall and gain entry from the garden.
Margaret Hall who had been baby sitting in apartment 5a the year prior to Madeleine's disappearance recounted witnessing a man lurking in the shadows ... we had some discussion whether that was at the front of the building or poolside.
http://miscarriageofjustice.co/index.php?topic=6209.msg232924#msg232924In my opinion her encounter was poolside.
Which certainly ties in with what Horrocks has said about the ease of access from that area.