Maryland decriminalized marijuana possession two years ago, making possession of a small amount punishable by civil fine.
But police still use the mere smell of the drug as the basis for searching vehicles.
The Maryland Court of Appeals heard arguments Thursday on whether that should change.
"The smell alone does not give rise to suspect a person is in possession of 10 grams or more of marijuana," attorney Ethan Frenchman told the court.
The appeal stems from three recent cases in which police officers smelled marijuana and cited it as probable cause to search vehicles where more drugs were found. The practice was upheld by trial courts and the Maryland Court of Special Appeals.
The General Assembly passed a law in 2014 that made possession of less than 10 grams of marijuana a civil offense that carries a $100 fine. The fine climbs to $500 for repeat offenses.
Lawmakers said they were hoping to end racial disparities in arrest rates for the drug and aiming to remove the employment and educational barriers that drug charges pose for thousands of Marylanders.
When the law took effect, some law enforcement officials wondered how the change might affect car stops, in which the smell of marijuana is often cited as grounds to conduct a search of a vehicle.
The first round of challenges in the appellate courts resulted in swift affirmation that the drug remained illegal and detection of any amount could be used to justify a search.
"Maryland has not legalized the possession of less than 10 grams of marijuana. It has simply ameliorated the punishment," Court of Special Appeals Judge Charles Moylan wrote in one opinion. "A 'civil offense' is not innocent behavior and the very word 'offense' should make that apparent. … The sanction of a fine is not imposed for behavior that is legal."
On Thursday, Frenchman compared a person with a small amount of marijuana to a fisherman in waters where catching 10 or more fish is banned.
"If you see a fisherman who's out fishing … and he smells like fish and you know he has some fish on him, is there any basis to conclude … that the fisherman has 10 or more fish?" he said.
Because possession of a small amount of marijuana is no longer a crime, Frenchman said, police should be required to cite some other factor to establish probable cause that a suspect has more than 10 grams.
"There must be some kind of information that gives rise to an identified suspicion that there's a criminal quantity of marijuana," Frenchman said.
Carrie Williams, an assistant attorney general arguing for the state, said marijuana remains an illegal substance in Maryland and is still subject to seizure and forfeiture in any amount. She said the General Assembly intended only to free up the criminal system from the burden of such cases, not legalize the drug.
"Where there's probable cause to believe a vehicle contains items subject by law to seizure and forfeiture, and marijuana belongs to that class, a search can be conducted without a warrant," Williams said.
Presence of the drug "is enough to provide officers with a fair probability that more grams of marijuana will be found in that vehicle," or that evidence of other crimes could be discovered, Williams said.
"Any amount is not innocent behavior," she said.
Frenchman agreed that marijuana may be seized, but said discovery of a small amount was not enough to take the step of conducting an "intrusive" search. He compared it to a juvenile smoking a cigarette, which can be seized by police, but is not grounds for a search.
One of the cases under appeal involved a search just two weeks after the decriminalization went into effect in 2014.
A Baltimore police officer said he was driving past a man leaning against a parked vehicle and was able to smell an "overwhelming" odor of marijuana. The man admitted he had marijuana in the car, prompting a search that turned up more than 10 grams of it, along with oxycodone pills.
"But no longer are law enforcement going to be allow[ed] to walk down a street corner, detect marijuana and arrest everybody in that street corner," the man's attorney argued, according to court records. "That's specifically what the legislature was concerned about."
The judge disagreed, as did the Court of Special Appeals.
In another case, police said they pulled up to a stop sign alongside another vehicle, approached the car and smelled marijuana.
At a hearing to argue over the suppression of evidence, the sides debated "the aromatic potency of fresh marijuana versus that of burnt marijuana and the relationship between the amount of marijuana at the source of the smell and the resultant distance at which the smell might be detected."
The appellate court said it was being asked to "assess the reasonable outer limit" of the detective's "olfactory power," but concluded the issue was moot: marijuana was and remains illegal in Maryland.
The Court of Appeals is expected to decide the issue in the next several months.