NBC

The idea that your home is your castle lies at the heart of the Fourth Amendment's protection against unreasonable searches.  So what if the police bring a dog to sniff for evidence at the castle door?

Two cases from Florida, to be argued Wednesday, ask the U.S. Supreme Court to decide when the police need a search warrant to use drug-sniffing dogs at a house, and how much legal authority a dog's alert gives police to search a car.

The front-door case comes from Dade County, where police received a Crime Stoppers tip that occupants of a house were growing marijuana.  After watching the house for about 15 minutes, police and federal agents sent for Franky, a drug-sniffing dog. 

A police handler walked the dog up to the front door, where Franky alerted the officer by sitting down after sniffing at the base of the door.  After using that result to get a search warrant, police entered the house and found marijuana plants growing.

But a judge threw the evidence out, ruling that "the use of a drug detector dog at the defendant's house door constituted an unreasonable and illegal search."  In other words, the court said, the police should have gotten their search warrant before they sent for Franky.

The Supreme Court has upheld the authority of police, acting without a warrant, to use dogs at airports for sniffing the outside of luggage suspected of carrying contraband or to sniff the outside of cars at roadside checkpoints. 

But the court has also said that police, without a warrant, could not stand on the street and aim a thermal imaging device at a house to see if marijuana was being grown inside with heat lamps. Such an intrusion, it held, would reveal the private activities of a homeowner, including such intimate details as "at what hour each night the lady of the house takes her daily sauna and bath."

The state of Florida argues that there's no violation of privacy in a dog's sniff at the door because drug dogs alert police only to the presence of illegal substances, something in which a homeowner has no privacy interest.

But the state's supreme court rejected that argument, finding that the Fourth Amendment's protections are at their highest at a house. It found the dog sniff to be "a substantial government intrusion into the sanctity of the home."

The National Association of Criminal Defense Lawyers urges the Supreme Court to affirm that ruling.  "Allowing suspicionless dog sniffs of houses would permit indiscriminate sweeps of residential neighborhoods, a practice that some law enforcement officials have already begun to employ," the group says in a legal brief filed in the case.

In the second case, a Seminole County deputy sheriff pulled over a pickup truck because it had an expired license. When he noticed that the driver was shaking and breathing rapidly, exhibiting behavior consistent with drug abuse, he asked for permission to search the truck.

The driver said no, so the deputy brought out Aldo, a drug-sniffing dog, from his patrol car to sniff around the truck.  Aldo alerted on the driver's door handle. Considering that to be sufficient cause to search the car, the deputy found chemicals commonly used to make methamphetamines.  The driver then admitted he bought them for that purpose.

The Florida Supreme Court threw out that evidence, too, concluding that there's no sure way to know exactly what caused the dog to alert.  "There is no uniform standard in this state or nationwide for an acceptable level of training, testing, or certification for drug-detection dogs," it said.

The Obama administration is urging the court to rule for the deputy and Aldo.  After all, the Justice Department argues, what a policeman sees, hears, and smells can often establish the legal justification for a search without a warrant.

Dogs, the government says, do it better. "An alert by a dog trained to identify certain odors provides an even stronger basis for probable cause to search a location for the odor's source," the Justice Department says in its brief supporting the state.