The Truth of the Lie film are not impressive and that the dogs are not infallible, I found them chilling. These two canines have an extremely high rate of accuracy. Time and expertise were invested in their training.
Also: The crime scene need not be "pure". Cavader dogs have detected the presence of blood and corpses even after cement has been poured on top of the site. Goncalo Amaral's critics have chastised him about 11 holiday makers renting out 5A after the McCanns, and prior to the two British trained forensic dogs being called in. The charge of incompetance does not hold water.
Another point; an extremely important one: The psychological profiler on the blog, Lies Our Mothers Told Us, who posts under the pseudonym, "Miss Represented", makes this astute observation regarding the Kercher murder case in Perugia, Italy : Consciousness of Innocence is offensive; the Guilty Mind tends toward defensiveness. In other words: The innocent person will reply to the charge, "We have detected cavader odor in the apartment and rental car used by you " with, "For God's sakes, what are you speaking of? That is madness, nonsense, impossible! Why would that be found, it defies reality!" whereas the guilty person will think of reasons: "We had meat packages and blood leaked; I was in touch with six corpses at work and brought my work clothes to wear on the trip; the cuddle cat needed washing; etc......"
Psychoanalysis is a HUGE part of forensic "evidence"; ask any FBI criminal profiler if their analysis is not vastly important to the prosecution.
Other scenes which were impressive in the documentary: The Smith family's siting of Gerry McCann carrying the "sleeping" child toward the beach at 10 pm on the night of May 3, 2007; the behavior of the McCanns when first seen by the Portugese police; the split-second scenes of re-enactment of the Tapas dinner.