In United States v. Johnson, 660 F.2d 21 (2d Cir. 1981), the defendant claimed that the drug dog's alert did not create probable cause to support the issuance of a search warrant for his luggage. Id. at 22.
He argued that the dog was "incapable of distinguishing between the actual presence of drugs in a container and the residual odor when the controlled substances are no longer there...." Id.
The Second Circuit rejected that contention as a misapprehension of the concept of probable cause.
It said, id. at 22-23:
"Appellant's argument with respect to the problem of a dog detecting only the residual odors as opposed to the drugs themselves misconstrues the probable cause requirement. Absolute certainty is not required by the Fourth Amendment. What is required is a reasonable belief that a crime has been or is being committed."