Chuzzle,
I'm sorry you have to go back to work in your condition. Times are tough these days. I wonder if everything will ever get back to how it was a year ago, but I doubt it.
Anyways, about your question. Xanax is a controlled substance, therefore it will show up in your system, no matter how little you take. What usually happens is the doctor performing the test will call you and ask you if you took that and then will want a prescription. Either the bottle with your name and info on it, or something from the doc stating that you take it. You don't have to go into details about why you take. They usually give you 3 days after that to get your information into their office. If after 3 days you fail to comply then you will have officially failed your drug test. They are looking for narcotics/controlled substance, not blood pressure or tylenol, or that sort of thing.
Hope that helped. Good Luck!
Chelle