That would probably have been a true positive, not a false positive. Your liver metabolises a small proportion of codeine into morphine, so if you'd been taking codeine for several days before the urine test, you almost certainly would have had morphine in your urine - only a small amount, but still above the detection threshold. Then again, if you'd used heroin 5-6 days before, there still could be a detectable trace of morphine in your body from that too: In cases of chronic use of high doses of heroin, I've seen urine tests come back with small traces (below threshold for "proving" use, but still detectable and measurable) 8-10 days after the most recent self-reported use.