How far back does mouth swab detect heroin?

Actually it is only effective for 16 hours the reason I know this is because a friend who is a community addiction worker rather indiscreetly told me this though they don't advise addicts of this. After being of gear for a week I used two days in a row and was screened 27 hours later and the result was negative. Although I only smoked two 0.3 bags in the preceeding two days it would still suggest that it was only effective for 16 hours. Dihyrocodeine I had taken a few hours before the screen did show I was also advised doses of buprenorphine under 8mg don't show.