Something I cannot get past is the fact that the use of a k9 is a form of search. Using a dog to determine if you have drugs in your car is a search and should be protected under the 4th amendment.
I think the reasoning on this is that a K9 sniff is "non-intrusive", thus it does not violate the 4th amendment.
Kind of like how a "Terry Stop" (pat down for weapons) is lawful. It is determined as "non-intrusive" as it's a simple pat down for weapons... not really a "search".