I have to go through Border Patrol checkpoints down here and the dogs are usually very good and a walk around takes about 2 seconds. However, it's absolutely not uncommon to see a false alert. It's happened to me twice when I had a ton of stuff in the car. The first time the guy was very professional about asking me to step out for a search and in 5 minutes I was on my way, but the second guy was a dick about it.
Favorite quote from that scenario was from my buddy:
"Sir is there anything in the car that might have alerted the dog?"
"No sir, all I can think of in there of interest to a dog is my Chik-Fil-A."
"The dog won't alert for food sir."
"Are you sure? Have you ever HAD Chik-Fil-A????"
"For me, my family, my friends and skiing, that is for me. That's my life. The joy I get from skiing... that's worth dying for." - CR Johnson
Slope Style Ski Shop, Main Street Breckenridge