The police didn't investigate the case as an abduction immediately, it was five days before they decided that's what had happened. That suggests to me that the zip was, like 5A's open window, or alerts by cadaver dogs, just an uncorroborated clue. Something else emerged after five days to corroborate the zip clue imo.
I would suggest that why the police went with the abduction theory in Cleo's case was because they had investigated ALL other possibilities - eliminated them - and followed the evidence which remained.
The striking result of the police tactic regarding pursuing the theory that Cleo had been the victim of STRANGER ABDUCTION is that by concentrating on that THEY FOUND CLEO alive and well.
I suggest that had they thrown all their resources at pursuing the proven false trail of parental involvement convinced that is what happened to Cleo to the exclusion of all else as in Madeleine's case - they would not have found her - and she would have been yet another child referred to in future as "another Madeleine McCann".
Cleo was found BECAUSE THE POLICE LOOKED FOR HER and never allowed the investigative trail to go cold as they checked EVERY possibility out - including waste bins.
I think they used the tried and tested search technique of starting at the centre and working out finding the missing child not in the proximity of the holiday area, but in the proximity of Cleo's home where her abductor had taken her.
In Portugal Madeleine was never given the benefit of the doubt that she was alive and should be looked for as a live and missing child.
The lead dectective is on record as declaring her dead immediately she disappeared and immediately blamed her parents with no further ado.
Thank God the Australian police actually looked for a live child 'cos guess what - that enabled them to find one.