Those 'on the spot' urine tests and mouth swabs are notoriously inaccurate, despite what the manufacturers might claim and should always be followed up by a lab test. When I worked with drug users this was a frequent bone of contention. Whilst many of them would swear on their own grandmother's life they hadn't touched anything although they of course had, there were actually a few who were trying to get off the stuff and they didn't take kindly to us stopping their methadone script on the strength of one of these tests, because it left them with no option other than to buy methadone on the black market, or go back to using opiates. Used to get really messy when they were trying to regain custody of their kids from Social Services care, as of course with a positive result they had no chance. Those results were (and still are) contested and are often proved to be false. Actually poppy seeds are known to produce a positive result as these tests are highly sensitive. Some prescribed medications can give a positive for amphetamine, any codeine-based painkiller will produce a positive for opiates too. The lab test, along with a list of any meds you might be on, should be reassurance enough for your employer.
