The timeline is,
We see Lea in Summer Knight, where she takes Harry to Chicago over Chicago, and warns him never to let Mab take him to the stone table. This may have been the event that triggered Mab's suspicions. Though that is speculation.
We next hear of Lea in Dead Beat. When Harry summons Lea , Mab appears wearing a knife that is assumed to be the Atheme. She tells Harry that Lea is detained but in no danger.
And the in Proven Guilty We see Lea and learn of the infection.
In Changes we see Lea when Mab brings Harry to the Stone Table to kill Slade and take the Winter Mantle. Lea reminds harry that she had warned him never to do this. Mab will draw blood on Lea when Lea speaks out of turn. All of this takes place in chapter 31.
“Yeah. But why?” I asked. “Why would your voice hurt me?”
“Because she is angry,” answered the Leanansidhe in her natural voice. “Because her voice is a part of her power, and her rage is too great to be contained.”
I swallowed. Mab had spoken a few words to me a couple of years back, and I’d reacted exactly as she described. I’d lost a few minutes of time during the episode her words had provoked as well. “Rage?” I asked. “About what?”
The shadowed figure let out a spitting hiss, another feline sound that made me flinch and cringe away from it as if from the lash of a whip. My godmother jerked sharply to one side. She straightened only slowly, and as she did I saw that a long, fine cut had been drawn across one of her cheeks. Blood welled up and dripped down slowly.
My godmother bowed her head to Mab, and the cold voice came from her mouth again. “It is not for my handmaiden to judge or question me, nor to speak for me upon her own account.”
Lea bowed her head to Mab again, and not a flicker of either anger or chagrin showed in her features.
Harry states his terms for accepting the Mantle. One of those is Mab's help at Chechen Itza.
“That before my service begins, you restore my body to health. That you grant me time enough to rescue my daughter and take her to safety, and strength and knowledge enough to succeed. And you give me your word that you will never command me to lift my hand against those I love.”
“There,” I said quietly to Mab. “My part is done. Time for you to live up to yours.”
“No, child,” said Mab’s voice through Lea’s lips. “Your part is only begun. But fear not. I am Mab. The stars will rain from the sky before Mab fulfills not her word.” She tilted her head slightly to one side, toward my godmother, and said, “I give thee this adviser for thy final quest, sir Knight. My handmaiden is among the most powerful beings in all of my Winter, second only to myself.”
Lea’s warmer, more languid voice came from her lips as she asked, “My queen, to what degree am I permitted to act?”
I thought I saw the fell light gleam on Mab’s teeth as Lea’s lips said, “You may indulge yourself.”
Lea’s mouth spread into a wide, dangerous smile of its own, and she bowed her head and upper body toward the Queen of Winter.
I speculated about the timing of Mab's suspicions of Lea due to Lea's remarks about the Stone Table in the two different Books. In the first she whispers it to Harry as if she doesn't want Mab to know,
Lea completed her circuit of the table and stopped beside me. She glanced furtively around her, then looked me in the eyes and said, her voice barely audible, "Child. Should you survive this conflict, do not let Mab bring you here. Never."
in the second while speaking for Mab.
“I warned you,” said a calm voice behind me. “Never let her bring you here, my godchild.”
As a side note unrelated to the topic this is the second time that we see that Mab, under some kind of condition, is capable of looking anywhere to see persons of interest to her.
