There is nothing illogical about my reasoning. Did the Tapas group leave their kids unattended most nights of the holiday? How difficult would it have been for an abductor intent on stealing a child to work this out, by day 6 of the holiday, either through observation, reconnaissance or tip-off?
I have also already conceded that maybe the abductor DIDN'T know that the children were unattended and entered the apartment on the off-chance, as has happened in other abductions and abduction attempts, also break-ins to the bedrooms of children holiday makers on the Algarve. I have said this all before on this thread so please don't claim again that I have not answered your question or get me to repeat it for the third time.
How difficult? By observation or recon? I can't prove it impossible, but it would not pass a test of 'beyond all reasonable doubt'. Unless you overcome your distaste for visiting Luz I will not be able to demonstrate this to you by first-hand experience.
By tip off? This requires that someone in the know is already aware of someone who wants to nick a child and is willing to pass on such information, presumably for cash though there could be a different motivation. I am trying to envisage the pub conversation that took place. Would it be 'hi, I've heard you are interested in nicking a kid, and I know where you can get one, if you have the required readies.'? Or would it be 'we both have an interest in nicking a kid, and I know where we can get one.'? That don't impress me much.
Opportunistic snatcher? Madeleine could have been snatched in such an incident. All that remains is means of entry and motivation. And lack of evidence. And those darn pesky dogs. The key word in your post re this point was 'maybe', thus sheer speculation.