Oh but I can. The apartment could still have been monitored and the abductor could still have found himself there at the worst possible time. Unless he had a time machine he was not to know that Gerry McCann would carry out a proper check immediately after Matthew had listened at the shutter. There was always going to be an inherent risk whatever time he or they chose as the movements of the checkers were so irregular.
A good point John. If they had been monitoring the McCanns and knew they checked approx ever 30mins then it's highly likely they thought that Matthew was doing the 30 min check for the McCanns - at their request.
They would not know that Matt's check was a spontaneous one which he only decided to do when he went off to hurry the Paynes up - and decided that as he was half way to his apartment anyway - he might as well carry on and do his own check, and while he was there he decided to check the McCann's as well.
If the abductors thought that was an arranged check they would assume that after Matt left they had 30 mins before the next check and so went in soon after Matt went back to the Tapas. The last thing they would be expecting was that Gerry would turn up within 10 mins.
IMO it's not completely out of the question that Madeleine could have already been sedated when Gerry last saw her and the abductor was hiding in the apartment. Then whilst giving Gerry a couple of mins to get off the road and back into the OC - he opened the window to check that the coast was clear at the front of the building and left via the front door with Madeleine.