From my past... hehe They can't "press charges" or whatever, for you testing positive. As a matter of fact, they can't even tell anyone else that you tested positive. You, the manager, and the lab should be the only people involved.
If you aren't an every day smoker, it's possible to be out of your system quick, but I doubt you will be clear from Yesterday til' today. Try to give it at least a week in the future, and about 18-22 days if you are an every day smoker
What makes you think they can't find out? I can't do it myself, but I know some people who could find out exactly who you are, using nothing but this posting, in less than an hour. I've seen them do it. And you just told a few million people what you did.