Don't think you can blame the McCanns or their friends for trampling the crime scene with dogs nor do I think they were spreading finger print powder all over the apartment.
"Initially the search began for latent shoe-prints it being verified that dozens existed on the floor, in the various rooms of the apartment, which invalidated the attempt of identifying those of the perpetrator. Also, innumerable tracks [footprints] that were taken to be canine in origin mixed with red- and white-coloured chemical products, as used to see fingerprints, and an enormous quantity of hairs probably of animal (dog) origin that made it difficult to find possible traces, especially in the bedroom of two single beds and two children's cots from
where the minor disappeared, and next to the aluminium window/door leading from inside the living room to the exterior area behind the apartment."
http://www.mccannpjfiles.co.uk/PJ/5A_FORENSIC_4_5_7.htm