The most fitting interpretation of the owl that Lady Macbeth claims to hear is:
Lady Macbeth actually hears Duncan shriek as her husband approaches him with his dagger.
In this context, the owl serves as an ominous symbol, representing death and foreboding. Lady Macbeth references the owl to indicate that the act of murder is imminent and highlights the severity of their actions. The "fatal bellman" is a metaphor for the owl's call, which aligns with the themes of death and dark omens present in the play.