In reports of the recent arrest and prosecution of Jared Fogle, it has come to light that one of the SD flash cards that apparently contained evidence of child pornography was located within his home during a search by police by a specially trained dog.
The police routinely use dogs to sniff for drugs and explosives, and because dogs have noses that are hundreds of times more sensitive than human olfactory sense, they can be trained to recognize certain odors and 'alert' for their handlers. This allows police to discover items hidden during a search authorized by a search warrant, as was the case with Fogle.
This is clearly a search, as police had a search warrant, and they were exhaustively going through the contents of Fogle's home. The dog's function was to extend the officer's human capabilities to locate electronics that could, and presumably did, contain thousands of illegal images.
Dogs are also used to sniff around vehicles when the police have no warrant, to detect drugs. The U.S. Supreme Court has idiosyncratically ruled that this activity is not a search, in a case named Place, from the early 1980s. However, it is difficult to distinguish how a dog's sniff is not a search in such context.
If they and X-ray machine to look inside the vehicle, the use of those devices would likely be seen as merely an extension of the officer's eyes, and as such, the use of the device would be viewed as a search.
The scent of drugs that a dog can smell are well below the human threshold of recognition, but merely because the dog is organic, and not a manufactured electronic device, should not make the dog any less of a search tool.
In some recent decisions involving dog sniffs, the Court appears to perhaps be slowly recognizing this theoretical problem. With dogs' roles in criminal prosecution expanding to encompass electronics, a better analysis whether their usage constitutes a search and under what circumstances would be helpful.
Source: cnet.com, "Electronics-sniffing dog found thumb drive with ex-Subway pitchman's porn stash," Chris Matyszczyk, August 29, 2015