Kagan, by contrast, minimized the significance of a dog's success at finding drugs in the field. She said police testing in artificial conditions is a better measure of reliability, even though handlers typically know where the drugs are hidden and can therefore direct the animals to the right locations, either deliberately or subconsciously.
Instead of requiring police to demonstrate that a dog is reliable, this decision puts the burden on the defense to show the dog is not reliable through expert testimony and other evidence that casts doubt on the training and testing methods used by police. But experts are expensive, and police control all the relevant evidence.
Police even determine whether the evidence exists. Many departments simply do not keep track of how often dog alerts lead to unsuccessful searches, and this decision will only encourage such incuriosity.
The court previously has said that police may use drug-sniffing dogs at will during routine traffic stops and may search cars without a warrant, based on their own determination of probable cause. Now that it has said a dog's alert by itself suffices for probable cause, a cop with a dog has the practical power to search the car of anyone who strikes him as suspicious.
Even the question of whether a dog did in fact alert may be impossible to resolve if there is no video record of the encounter, which is often the case. As Florida defense attorney Jeff Weiner puts it, the justices "have given law enforcement a green light to do away with the Fourth Amendment merely by uttering the magic words, 'My dog alerted.'"