Sadly enough, you can fail a drug test from " secondhand smoke" ( technically, only the smoke that hasn't been inhaled contains enough THC to cause anything) so if he was, say, in a car with someone burning a fatty, he could have garnered a small percentage of THC in his bloodstream. But on the other hand, there would have to have been a significant amount of undiluted smoke in the car, and the drug test would have had to occur in something like 12 hours after the instance for that to be true. So it could go either way.