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Of course she didn't know. That's the point. She's a whore.
She was ready to show her boobs to anyone who would be outside the door.
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Liam was very attentive and loving the morning Sophia met him in the kitchen. He told Sophia that she was a fool to miss out on a wonderful evening yesterday and that she looked incredibly sexy in her dressing gown.
These words and his behavior are the balm on Sophia's ashamed soul. Sophia has the feeling again that everything can still go well and that she has retained control.
But Liam has to go to work and leaves the house. When the doorbell rings shortly afterwards, Sophia believes Liam has come back because he has forgotten something and wants to continue what started between her and Liam in the kitchen.
Better explanation for current behavior.
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I think, their behavior will match your simple statement later in the game.