Listen, I once had to take a piss on the road, so to be discreet I stood very close to my minivan .... yeah this was a long time ago when minivans were not quite so uncool .... and some of my pee splashed on the tyre. Hours later when I got to the border one of their dogs picked up the smell on the tyre and they ripped my vehicle apart. Found nothing of course but I was very pissed off. I have successfully smuggled weed many times in the past and you have to take extra special precautions. The only things the smell will not pass are metal and glass. Anything else is an invitation to get caught by the sniffer dogs.