Mab might want to mention to Elsa that she wants that particular secret kept. She has and does speak to Sylvia and that's information that Sylvia would probably want. But Elsa might be willing to keep her secret. For what reason Elsa is not entirely sure.

"It's hosted by a human. Most of us here are human, I believe." She ascends the staircase, warily, definitely not trusting Mab but hoping the fae wouldn't choose to start something at a party.