So the secret is the note? She’s ashamed that she talked to kristin.

I like the smell of cigarettes too.

I would say that her overt secret is her admitting she fakes smoking, since it’s part of her image. However, a girl like Kristin is the supposed antithesis of Maria — an upstanding young citizen compared to her archetypical bad girl. The fact that there is an admiration there, a desire, a yearning to have that life is something that Maria harbors deep within her, sharing with nobody until now.