Showtime,
As we all know, nothing is infallible. Something is causing you to test positive for benzos-they don't just appear in your blood from no where. Take Robert's advice and question your doctor about your meds. If you don't like his answer or just want to hedge your bet, ask your pharmacist, too. Either way, make sure if someone can tell you which one could be causing it that you have that person talk to your outpatient therapy clinic.
Unfortunately, it's things like this that can cause people to want to give up therapy. I hope you stay with it and good luck.