I passed a drug test for this job I was trying to get and got a congratulatory email from the HR guy. A little while later I was at a bbq that my coworker was having and we were talking about this HR guy (who everyone HATED) and I told them about this email. They all told me none of them got this congratulatory email despite the fact that they all went through the same exact process with the same exact people. I think the guy expected (maybe wanted) me to fail the test. Little did he know I hadn't smoked in a long time because I was broke because I had no job.
This same guy later fired me because I took vacation for a month without approval. He called me when I was at the airport telling me I was fired and I think he really enjoyed that.
Good luck on your test, if you have it.