Yes the false positive is possibility. If you are clean and the doc doesn't believe you, tell him that it can be verified by Mass Spectrometry (Mass Spec for short) and Gas Cromatography. Although I doubt he would actually go to all that trouble.
Found this when doing some research for you.
The active narcotic in the Suboxone is Buprenorphine, which is a synthetic opiate and, on the non-certified tests that are much less reliable, both the synthetic and semi-synthetic opiates have been known to cause false positives for their relatives. And, in this case, Oxymorphone is another synthetic opiate.
This has happened to many people, because the doctor's offices are not required to use certified tests, thus they usually use a lab that was the cheapest bidder for their business. On these types of tests, false positives can occur anywhere from 4 to 50% of the time.