The Court of Appeal holds in this case that it's only a "de minimus" search -- and hence okay under the Fourth Amendment -- for a police officer (without a warrant) to insert a key into a residence to see whether it unlocks the door.
You can see where the Court of Appeal's coming from. Though you can certainly imagine the contrary arguments as well.
Imagine that this same case, however, came to the Court of Appeal in a different factual setting. One in which a defendant was sentenced to six years in prison for "burglary" based upon his insertion of a key into another's residence, and the prosecution argued that he was guilty given the time-honored rule that any entry into a residence -- "however slight" and even by a tool (e.g., a key) -- resulted in criminal liability. My strong sense is that, in such a case, this panel would not view the entry as de minimus but would instead uphold the imprisonment of the defendant for many years.
But what's illegal and unreasonable for individuals is perhaps legal and right for police officers.