What Causes Positive Test Results For THC In A Non Smoking Person?

Hi, I just did a drug test in an ecup and failed for THC, thing is i have not had a smoke since October and it is now January. I smoked everyday for 2 weeks then completely stopped for almost 3 months now, how is this possible?. Also the meth and benzo lines were faint and i have never touched those drugs in my life, they said it was a bad batch and those lines were always faint.