Might not be looked for in the toxicology tests.
Crude opioid tests would not be able to discern between this and the levorotatory opioids which derive from the opium poppy. More sophisticated ones would, and might miss it. At any rate hopefully someone at the Sanford coroner kept a blood sample for any future questions of this kind.
But maybe Tray was running dry of this stuff which is why the still intact Skittles and watermelon drink, in preparation to adding it to some Robitussin later. Nothing was said by the reporter about whether the psychotic break from large doses of dextromethorphan would last, or whether it goes away when the drug wears off. There are still a few gaps, but it suggests more trouble waits for Tray defenders.