Don't worry about it. Number one, drug tests are not 100% accurate, and not even 90% accurate unless they do bloodwork, which is expensive. Most times they give you a "do-over" as it were, since certain things in your system can give a false positive. (Poppyseeds, for example, will give a false positive for heroin) Beyond that even if you fail it the second time it's really none of their concern, they just won't hire you. They won't notify the police because the police would have one hell of a time making anything stick in court. ("Marijuana in my bloodstream? Well I went to a Tool concert a week ago and I couldn't see across the arena...") It does tend to look a little bad for you though, and if it's a government job it can travel through the grapevine a bit even though it's not supposed to. Or so I hear.