Hello, I Have A Question And Hopefully You May Have

Hello, I have a question and hopefully you may have an answer for it. My 16 year old step-son is a wonderful, straight A, sports playing kid. He recently applied for a job at the local pizza place and they gave him a drug screening. A week later, Moma Rosa Pizza Place, called to tell us that he failed his drug screening because of Heroin! I told them that the test was most definitely not true. There is no way that our wonderfu son is on any drugs whatsoever. Please help me to understand what would make a drug screening show something that is not true!