How could Snape have erroneously suspected Dark Magic in the Marauder's Map?

Question

In Prisoner of Azkaban, when Snape examines the Marauder's map and gets insulted by it, he immediately calls Lupin and tells (and asks) him about it. He also claims that it is plainly full of Dark Magic:

“Well?” said Snape again. “This parchment is plainly full of Dark Magic. This is supposed to be your area of expertise, Lupin. Where do you imagine Potter got such a thing?”

-Prisoner of Azkaban, Chapter 14: Snape's Grudge

Now, it is hard to believe that the Marauders ever had anything to do with the Dark Arts, let alone use them in the map. It has been well established that James hated them, so it is safe to assume that the others were no different. In fact the magic used in making the map is already known: It was the Homonculous Charm.

On the other hand, it is also quite abstruse that Snape asserted the presence of Dark Magic in the map; being exceptionally skilled in Dark Arts, it is highly unlikely that he could go wrong, unless he wished to use that assertion to accuse Lupin (the DADA teacher) of giving the map to Harry. But that wasn't the real reason he knew Lupin was related to the map; he knew that because he had recognized the code names on the map, as this answer proves.

How could Snape have indeed been mistaken? And if not, what was his real motive for his claim?

Answer 1

The answer you cite makes it clear that Snape brought the map to Lupin because he knew Lupin was one of the "manufacturers." He could infer quite a bit about the map, but he couldn't say as much as he knew, because Harry was right there. Saying "and it's got your name on it: 'Moony,' that's you, isn't it?" would be bringing up a whole raft of embarrassing memories in front of Harry, which simply isn't an option for Snape, emotionally.

His claim that the map is "plainly full of Dark Magic" is somewhere between a bluff, a veiled insult, and a straight-up lie. He's trying to "make Lupin sweat," as Jason Baker said in comments; he's trying to mislead Harry; and in a very roundabout, damaged way he's trying to protect himself from what the map represents.