c'mon man, enough with the what-ifs. This post is about K-9 units violating the 4th amendment and you want to sideline the discussion with some off the wall, wilder than possible scenario. If the fire just happened 5 minutes ago, how does Barney Fife know an accelerant was used? I got gas on my shoe filling up my car this morning. Should I be stopped by every cop I walk by and questioned? I mean, who knows, there could be a fire a few blocks away that hasn't been reported yet. So here is my "line":
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
So now try to stuff that square peg into the round hole you created with your "what if" scenario. So, yes, you do have to catch someone in the actual commission of a crime or have an affidavit of someone who did. SHOCKING, I KNOW!!!!
Doesn't much matter, though, since the cops will stop you and anyone else they want and ask whatever questions they want anyway.