Yvette kills the cook and runs back to the billiard room where she starts screaming. You don't hear the cook scream because Mrs Peacock is also screaming. After hearing Yvette scream, they all run to the billiard room. So far so good. When they ask Yvette why she was screaming, she replies that she also drank the cognac. However, she was busy in the kitchen killing the cook when Mrs Peacock started screaming about the drink being poisoned. So how could Yvette possibly know about Professor Plum's theory of the drink being poisoned?