Still not clear on type of test.
In short, it is known that unanticipated conversions between opioids may occur. One of the most noted is an oxycodone false positive from hydrocodone.
Hydromorphone (Dilaudid) is also a breakdown product of Hydrocodone and Percocet contains oxycodone. This sequence could explain the false positive.
It always attracts my attention when it is stated that Percocet is detected because that isn't the case. It is only oxymorphone a breakdown product of the oxycodone contained in Percocet that is identified.
While these tests are procedurally simple (that's why they're used) proper interpretation of the results requires a thorough knowledge of opioid pharmacokinetics and the vast potential for cross-reactions.
Following is a resource that explains the processes more completely:
Pain Treatment Topics