But the point is that a dog sniff is not a "search." One cannot have a reasonable expectation that illegal activity cannot be discovered by government conduct that discloses solely that activity and nothing else. I'll just excerpt more from the opinion:

We have held that any interest in possessing contraband cannot be deemed “legitimate,” and thus, governmental conduct that only reveals the possession of contraband “compromises no legitimate privacy interest.” This is because the expectation “that certain facts will not come to the attention of the authorities” is not the same as an interest in “privacy that society is prepared to consider reasonable.” In United States v. Place, we treated a canine sniff by a well-trained narcotics-detection dog as "sui generis" because it "discloses only the presence or absence of narcotics, a contraband item." Respondent likewise concedes that “drug sniffs are designed, and if properly conducted are generally likely, to reveal only the presence of contraband.” Although respondent argues that the error rates, particularly the existence of false positives, call into question the premise that drug-detection dogs alert only to contraband, the record contains no evidence or findings that support his argument. Moreover, respondent does not suggest that an erroneous alert, in and of itself, reveals any legitimate private information, and, in this case, the trial judge found that the dog sniff was sufficiently reliable to establish probable cause to conduct a full-blown search of the trunk.

Accordingly, the use of a well-trained narcotics-detection dog – one that "does not expose noncontraband items that otherwise would remain hidden from public view," – during a lawful traffic stop, generally does not implicate legitimate privacy interests.