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Originally Posted by MysticCat

Actually, the reference to it is at the end of Goblet of Fire. In the hospital wing, after Snape showed Fudge the Dark Mark and Snape and Sirius reluctantly shook hands, Dumbledore said "'Severus . . . you know what I must ask you to do. If you are ready . . . if you are prepared.' 'I am,' said Snape," and with a "good luck" from Dumbledore, he left. Then at Spinner's End, Snape explained his failure to respond immediately to the Dark Mark by telling Narcissa and Bellatrix, "By waiting two hours, just two hours, I ensured that I could remain at Hogwarts as a spy . . . ."

So he is undercover all through both OoP and HBP, but no mention of it is ever made in OoP.

Thanks, MC - I guess I wasn't thinking of "undercover" in that manner