Mike, by left the doors open do you mean unlocked or open as in not closed?
My understanding is that the McCanns had always left the patio door which leads out on to the veranda unlocked (but pulled too) when they went out to wine and dine at the poolside tapas bar, and that they did not alert anyone to this fact, and no-one amongst their group suspected them of doing it, until just before 9:30pm, on the evening that Maddie went missing. This came about because one of the volunteers asked for the key to the apartment and he was told by Kate McCann that there was no need for use of such a key because the patio door on the pool side of the apartment (which could just about be seen from where the McCanns were situated) had been left unlocked and could be slid open? You then had the check made by the two volunteers, and the contradictory accounts about whether they went into the apartment, or listened at the patio door? One thing is for sure and that is that Maddie was not taken from the apartment by anyone using the patio door visible from the poolside tapas restaurant...
Now...
If Maddie had already been taken by the time these two volunteers offered to go and check the McCann apartment at about 9:30pm, how strange that any would be abductor would know not to use the unlocked patio doors on the poolside of the building, considering that no-one knew this to be true, until Kate told one of the two volunteers at about 9:30pm, and that as a result of this information being passed by Kate to the volunteer, that he should go along to the apartment with another and return saying that everything was tickety boo, with the McCann kids at that time? Oddly, the other volunteer did not return back to the tapas bar, until much later, which begs the question did this volunteer play any role in Maddies disappearance? If Maddie was in bed inside apartment 5a at the time of the 9:30pm check, then one of the two volunteers was away from the tapas bar at about the right moment when Maddie was taken after the so called 9:30pm check...