I've read that these dogs had some tracking training. It is fairly instinctive in some dogs. I'm sure we've cited this already.
That was the PATROL dog used at 02.30am on 4th May. [cite already given]
The two TRACKER dogs, Rex and Zarus, arrived at 11pm on 4th May from Lisbon with their handler Officer Fernandes. They immediately used the dogs who were given the towel and who both went down the path etc. They were used again on 8th inside the two blocks, with the same item, and again on 10th when all apartments to which OC had keys were opened. The discussion on 7th or 8th May with the PJ was about the likelihood of them being able to track given the time which had passed.
At about 19.. the undersigned officer, accompanied by the Commander, Officer Silva, took part in a meetings with the PJ Directorate, being asked by the PJ about the viability of giving the girl's clothes to the dogs for the dogs to sniff again, and if by means of the odour inhaled, they would be able to mark an identical odour in one of the resort apartments even though its door was closed.
With regard to this task, Officer Silva referred to the fact that the time that had passed would be a crucial condition for the dogs' work in obtaining results and that the entirety of the human odours existing in the apartments and access paths could make the dogs' searches very difficult. However, in spite of not being a normal situation for tracking, it could be attempted, whilst the operation should be carried out as quickly as possible and not directed towards one but to all the apartments in the resort, it being appropriate for the handler not to know which apartment was chosen, so as not the be conditioned.
http://www.mccannpjfiles.co.uk/PJ/GNR_SNIFFER.htm