By Jonathan Stempel 

WASHINGTON, Oct 31 (Reuters) - While considering whether a police dog should be allowed to sniff outside a home for illegal drugs inside, some U.S. Supreme Court justices smelled a rat.

At their Wednesday session, justices from across the ideological spectrum signaled that the privacy interest of a person in his home was too great to give police a broad license to let trained canines sniff around a home for evidence they could not see.

But in a second case involving a sniffer dog, some of the justices indicated they were hesitant to set too high a bar on police to show that their dogs are reliable.

The nine-member court has often allowed dog searches, including of luggage at airports and cars at checkpoints.

On Wednesday it addressed Florida's appeals of two decisions by the state's highest court that found the detection of drugs by trained police dogs violated the ban on unreasonable searches and seizures under the U.S. Constitution's Fourth Amendment.

The first case focused on the location of the search, on the doorstep of a home, while the second was focused on whether the dog in question was sufficiently reliable.

REASONABLE EXPECTATIONS OF PRIVACY

In the first case, police let their chocolate Labrador retriever Franky sniff the base of the front door of Joelis Jardines' home near Miami after receiving an anonymous tip that marijuana was growing inside. Franky's "alert" led to the discovery of more than 25 pounds of marijuana inside.

At least six justices challenged some assertions by Gregory Garre, a former U.S. solicitor general arguing for Florida, who said drug detection dogs only reveal the presence of contraband, in which no one had a legitimate expectation of privacy.

"That just can't be a proposition that we can accept across the board," Justice Anthony Kennedy said.

Justice Antonin Scalia said it would be okay to let police use binoculars to look inside a home from afar if the blinds were left open, but not to walk right up if they saw nothing.

"Why isn't it the same thing with the dog?" Scalia asked. "It seems to me crucial that the police officer went up to the portion of the house as to which there is privacy."

Garre said police deserved the capacity to effectively combat the "serious epidemic" of so-called grow houses.

Justice Stephen Breyer, however, said many homeowners would resent having a dog walk up and down near their homes. "You're looking at the expectation of a reasonable homeowner," he said.

Some justices likened Franky to the thermal imagers that the Supreme Court said in 2001 could not be used to look inside homes, because they could uncover things that deserved privacy.

Justice Elena Kagan asked if police could use a "Smell-o-Matic" that found the same things a dog might find. "Your basic distinction is between a machine and Franky," she told Garre.

Howard Blumberg, an assistant public defender arguing for Jardines, the homeowner in the case, also came under fire.

Justice Samuel Alito rejected as too broad his argument that Franky's sniff was a search because it revealed details that a homeowner wanted to keep private. Blumberg also called the sniff a trespass, but struggled to tell Alito whether any cases in the last few hundred years had made that point.

Chief Justice John Roberts asked if it mattered that mothballs, which mask odors, were found outside Jardines' home.

"Are we talking about an expectation of privacy in the marijuana or an expectation of privacy in the odor?" he said.

ADEQUATE TRAINING

The second case concerned the discovery of methamphetamine ingredients inside Clayton Harris' pickup truck, after it had been pulled over for having an expired tag.

An officer gave his German shepherd Aldo a "free air sniff" after the nervous-looking driver refused to allow a search.

Florida's supreme court said the state did not show enough evidence, beyond training and any certifications, that Aldo's nose was reliable.

Glen Gifford, another assistant public defender arguing for Harris, said more evidence was needed, and that dogs' enforcement records and the conduct of their handlers might also play roles.

But he couldn't offer what Roberts called a "magic number" for the percentage of correct alerts that would be acceptable to determine whether a dog was reliable.

Scalia challenged the argument that police might deliberately use ill-trained dogs to generate more false alerts, and more searches.

Police "like to search where they're likely to find something, and that only exists when the dog is well-trained," Scalia said. "They have every incentive to train the dog well."

Decisions are expected by the end of June.

The cases are Florida v. Jardines, U.S. Supreme Court, No. 11-564; and Florida v. Harris, U.S. Supreme Court, No. 11-817.

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