Can you figure out this “puzzling” case of oxycodone overdose with a negative UDS?

I was asked informally by a non-psychiatrist physician about the following clinical case that had puzzled the entire treatment team. The patient had a history of misusing oxycodone but had abstained from that for a few months. Then, he was found unresponsive at home. When the paramedics arrived, they gave him intranasal naloxone, and he…