Exactly. A piece of cake in fact. Excepting of course that no one was expecting an abduction. Well, you don't, do you. Although most of us would nowadays, because it happened.
There is no excuse for complacency when on holiday. Parents must keep their guard up at all times.
Anyone could have sneaked into the McCann's holiday apartment that night completely unseen. In fact somebody did if one chooses to believe the abduction from the bedroom theory. Personally I still believe Madeleine was taken from the street outside after she escaped from the unsecured apartment via the patio door.
We know that Gerry McCann stood conversing with Jeremy Wilkins on the street below the patio steps a short time after checking on the children. Madeleine could well have been disturbed as he left the apartment and heard them chatting outside and decided to pursue for all we know.
The sad thing though is that we may never know what actually occurred or if Bruckner was involved at all to any extent.