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.