Police have long used drug-sniffing dogs in the war on drugs. A dog’s much-more-sensitive olfactory powers allow it to detect the presence of drugs inside of vehicles and luggage that law enforcement would otherwise need a warrant to search.
The U.S. Supreme Court ruled in the 1980s that a dog sniff is not a search, so is not subject to Fourth Amendment requirements. However, a properly trained dog can provide police with the probable cause they need to then conduct a legal search. However problematic the underlying logic, this has been the law.
It works because the dog is trained to alert to the scents of drugs like cocaine, marijuana, heroin or methamphetamine, and the possession of any amount of these drugs has been illegal. But for a dog trained to sniff drugs that include marijuana, an alert outside a vehicle in Colorado or Washington would no longer provide probable cause for a search, because the dog cannot indicate which drug they had detected or the amount.
As more states lessen the restrictions on marijuana possession, it could cause further problems for any law enforcement agency that attempted to use a dog that had been trained to detect marijuana, as it is costly and time-consuming to retrain a dog to "unlearn" a drug scent.
How quickly the police adapt to the new realities in these states is unknown, but if past practice is indicative of future performance, it is likely that law enforcement will violate the fourth Amendment with sloppy searches, and continue to attempt to use dogs inappropriately in situations where they can no longer provide valid probable cause for searches.
Forbes.com, "What Good Is A Pot-Sniffing Dog When Pot Is Legal?" Jacob Sullum, January 15, 2015