So the McCanns left the patio door open, the other door was apparently dodgy they had the apartment that was nearest to the road so were in a vulnerable position as to an opportunist thief or even "abductor".
The Paynes had a baby monitor
Tanner and O'Brien had a baby monitor
so that just leaves the Oldfields, the Oldfields had the apartment next to the McCanns, so it wasn't down a dark little path (none were) it was adjacent to the McCann apartment in the same block adjoining the McCann apartment, The Oldfields had a patio door, they locked it at night according to Rachel Oldfields Rogatory statement
http://www.mccannpjfiles.co.uk/PJ/RACHAEL-OLDFIELD-ROGATORY.htm
Matt Oldfield stayed in the apartment on the sunday evening
Rachel Oldfield stayed in the apartment on the Wednesday evening
On the Tuesday or Wednesday evening Russell O'Brien stayed in the apartment because his daughter was ill
Strangely, all the Tapas group dined out at the Millenium Bar on the first evening, the saturday - with the children, but that never happened again, instead, they
all decided to leave their children in the apartments for the other nights.
Personally I find that odd, that every couple should decide to leave the children every other night, even though we know some of them had diarrhoea, were sick, had woken up crying, had mentioned that the parents hadn't come when they were crying etc, yet not one of the couples decided to take their children to the restaurant with them?
SO in fact, the only times according to statements that every couple was at the Tapas bar were monday Thursday and one of tuesday or wednesday evenings (due to RoB staying with his daughter).
Of those two or three evenings, only the McCanns and the Oldfields didnt have a baby monitor and the only couple who left the doors open were the McCanns.
The apartment that was least secluded was the McCanns as it bordered the main road and the street lighting, but the other apartments were all in the same vicinity, none of them were down dark alleyways they were all within one apartment of each other, only the Paynes were not on the ground floor but were on the first floor above in between the Oldfield and Tanner apartments.
There is a photo here about threee quarters the way down the page of the apartment locations.
http://www.gerrymccannsblogs.co.uk/APARTMENT.htm
According to the statements each couple checked their own children, the McCanns never checked anyone elses children but apparently Oldfield and Payne checked the McCanns children.
Again according to the statements then, sunday and wednesday, the Oldfields would not have checked the children because one of them was with the children
Tuesday or Wednesday O Brien was with his children
so potentially only monday and thursday would have been evenings when all the Tapas friends were out together, if "an abductor" had been watching, they would not have known the reasons why not all the group were out on certain evenings which makes it a bit of a stretch to suggest that "the abductor" could have been watching the movements for some time, because there was in fact virtually no pattern of events