Under Illinois v. Caballes, 543 U.S. 405 (2005), the use of a drug sniffing dog to alert for drugs is not a Fourth Amendment search. Caballes reasoned that possession of the drug is a crime, and a sniff that only alerts to the presence of the drug does not infringe on an expectation of privacy that society is prepared to recognize as “legitimate.” A student of mine raises an interesting question: Is use of a drug-sniffing dog a “search” if the dog is trained to alert to marijuana but the state has decriminalized marijuana? Granted, even if the dog alerts, the alert may not amount to probable cause if marijuana was decriminalized. See Commonwealth v. Daniel, 464 Mass. 746, 985 N.E.2d 843 (Mass 2013). But as far as I know, no court has considered the antecedent question: Where the police uses a dog that alerts to the presence of marijuana, is the use of the dog itself now a search? In other words, does decriminalization undercut Caballes? And does it depend on whether the dog is used by state agents (who work for the sovereign that has decriminalized marijuana) or by federal agents (who work for the sovereign that has not), or whether the state has decriminalized only certain quantities of possession? In Washington, the police are retraining dogs to no longer alert to marijuana, so perhaps changing law enforcement practices will ensure that this issue doesn’t come up often. Still, it’s an interesting question for us Fourth Amendment nerds.