Mr.Bungle

My first post, btw- been lurking for a while;

I'm on my way home from work on a 2 lane with my coworker in the passenger seat. I pull up to a red light next to a Police Crown Vic and the K9 unit in the rear seat starts barking and lunging at the window directly towards my car, to the point where the entire Crown Vic was shaking. Now I have no doubt the inside of my car reeked of pot. Thankfully me and my coworker burst out laughing instead of trying to pretend we didn't notice anything (which might have looked a little suspicious). The question I have is aimed towards the LEOs on the board but all responses are welcome: If a K9 is trained to be a narcotics sniffer, would it just automatically alert whenever it smells something or does have to be speciffically told by its trainer to go into "on mode."