I think part of the problem may well have been over reliance on an interpretation of the expert advice which was based on a total misinterpretation of what they were being told.
The Interim Report which Mr Amaral used extensively in his book illustrates perfectly both a misunderstanding of custom and practice and the role of the dogs in an investigation.
**snip
A report by Chief Inspector Tavares de Almeida to the Coordinator of the Criminal Investigation
( convicted of torture ... http://miscarriageofjustice.co/index.php?topic=2140.0)
Where he concludes, after analyzing all the evidence gathered, that the child is dead and the parents were responsible for cadaver occultation, and the entire GROUP was lying since the first day of the investigation.
http://www.mccannpjfiles.co.uk/PJ/TAVARES_ALMEIDA.htm
**
For example he laboured under the impression that the dogs had 100% success rate. He thought that if the dog alerted that was verification that human remains had been present in that location with a total disregard for the need that such an assertion must be backed up by evidence.
All of those assertions were nullified by proper interpretation of the FSS reports. So there are experts and there are experts and some of the danger lies in thinking the expert has told you what you want to hear not what was actually said.
What evidence did Almeida actually refer to? It seems the alerts were just the final bit which convinced the investigators that their already existing suspicions were correct.
1. The fact that the parents insisted on the abduction scenario from the beginning, and publicised their opinion using the media.
2. The whole group appear to have agreed together on the frequency of checking. Their statements are so incoherent that they all appear to be lying.
3. Jane Tanner's sighting is not very convincing, particularly as Gerry didn't even see her. Her description of the man kept changing also.
4. Simple things became disinformation: the question of the opened or closed window; the shutter up or down; the balcony door opened' the front door, locked or open.
5. Nevertheless, before the Media they kept (and keep) declaring their hope on finding their daughter alive: the first time that the hypothesis of the death of the little girl was raised it was, effectively, suggested by the McCann. (Krugel)
6. Although the parents publicised how much they wanted to help the investigation, most of the above points demonstrate how, in fact, they were spreading confusion.
7. The story was changed constantly in order to explain facts as they emerged.
8. Publicity was insisted on despite warnings that this could lead to the child being harmed.
9. Simulation of the scene was suspected.
The work done by the dog team, in Portugal, brought conviction of this slope [ angle] / proposition [ theory] of the investigation, not that it had been ignored previously, but because they [the dogs] could have determined the place where clues / trace evidence could be recovered that could be, scientifically, proven.
http://www.mccannpjfiles.co.uk/PJ/TAVARES_ALMEIDA.htm