Okay, so I waited to see if someone else would guess it, but...
It's Lois, to Clark, in IGACOY (when he blows her cover and then points out that he can still go undercover at the Metro Club).
Right?
Lois: You know, I have a funny feeling that you didn't tell me your biggest secret.
Clark: Well, just to put your little mind at ease, Lois, you're right.
Ides of Metropolis
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