Just did a little google and found:
Cannabis and its active ingredient THC is tested for in most drug tests. After one-off or occasional use its break-down products or metabolites can be detected between 3-5 days after use.
However, THC can be detected in a chronic user up to 12 weeks after use although the average is 25 -27 days.
Cannabis is highly detectable a long time after use because THC lingers in the fatty tissues of the body and leaks steadily into the blood and then the urine over weeks.
False positives have been known to be triggered by consuming hemp-seed bars and other products, although the more detailed, more expensive gas chromatograph-mass spectrometer (GCMS) test can tell the difference.
In rare circumstances, passive smoking can cause false positives in urine tests, up to 4 days after exposure, although those who passively smoke have noticeably less concentrations in their urine.
There is some controversy over whether the cut-off levels used to detect cannabis in blood and urine tests is too low, leading to a high degree of false positives. If you feel your drug test is false due to passive smoking, then you could demand that the threshold of your test is 65ng/ml, not the 15ng/ml of many laboratories. The higher level is thought to distinguish between active and passive smoking