They weren't undercover. They were agents on Robin's trail that likely had been for years now. If they have an opportunity to pluck her from the group discretely, it would probably be easier than just summoning a war between marines and the whole crew together. I said they were CP agents, not CP9. They seemed too generic too be CP9 anyway.

Or maybe, as Chopper suspected, they were just kidnappers, since half the archipelago is lawless and a lot of bandit groups exist there.