The Tampa Bay Times reports that the Pinellas County Sheriff's Office has been using a surveillance camera mounted on a pole outside Simply Hydroponics, a store in Largo, Florida, to identify possible marijuana growers. The operation has generated at least 39 search warrants in the last couple of years. But buying hydroponic equipment is not enough to establish probable cause for a search, since the equipment has many legal uses. To get inside the homes of the store's customers, deputies sometimes resort to highly implausible claims about clues they supposedly detected while standing outside.

On May 25, 2010, for instance, detectives showed up at the St. Petersburg home of John Ray, whose vehicle had been seen at Simply Hydroponics a year before. Lacking evidence for a warrant, they asked him if they could search his house. He said no. They forced their way in anyway and found a single plant in the garage. (Ray, who denied shopping at Simply Hydroponics, told the Times a relative who had stayed with him may have left the plant there.) The detectives later claimed that, standing at Ray's front door after he refused to let them in, they smelled marijuana and heard "the distinct sound of foliage being broken." 

Another example:

In the case of Allen Underwood, [a] Seminole man deputies spotted at Simply Hydroponics in 2010, detectives said they twice smelled marijuana from a sidewalk that was 15 to 20 feet from his home. Underwood's attorney, Jerry Theophilopoulos, says the sidewalk was nearly 70 feet from the alleged grow room.

The Times throws some cold water on such claims:

Some experts say it is improbable, if not impossible, for officers to smell marijuana growing inside a closed structure.

Dr. Richard Doty, the director of the Smell and Taste Center at the University of Pennsylvania, has conducted experiments to test whether people are able to smell marijuana from outside a home where plants are being grown. Insulation and ventilation of the home, the maturity of the plants and the distance between the nose and odor source can affect that ability.

Young plants don't emit an odor, but if a person were growing "hundreds" of mature plants in a structure that wasn't properly sealed, it's "possible" an officer very close to the home would notice the smell, Doty said.

James Woodford of Chattanooga, Tenn., an expert on the topic of marijuana odor, said a large operation vented directly outdoors could generate an occasional "whiff" of marijuana detectable up to 25 to 30 feet away [i.e., less than half the distance between the sidewalk and Underwood's "alleged grow room"]....

Law enforcement officers commonly use the smell of marijuana to establish probable cause. "Many such claims defy common sense, even though the courts routinely accept them as truth," Doty said.

Last May I discussed a Supreme Court case in which police looking for a crack dealer broke down the door of an apartment based on the smell of marijuana and the always-suspicious "sound of persons moving." The method used by the Pinellas County Sheriff's Office to find pot growers was pioneered by the Drug Enforcement Administration in the late 1980s, when the agency's Operation Green Merchant started tracking buyers of hydroponic equipment. 

[Thanks to CK for the tip.]