‘My mother must have been praying for me at that moment,’ were the prisoner’s words at the preliminary inquiry, and so he ran away as soon as he convinced himself that Madame Svyetlov was not in his father’s house. ‘But he could not convince himself by looking through the window,’ the prosecutor objects. But why couldn’t he? Why? The window opened at the signals given by the prisoner. Some word might have been uttered by Fyodor Pavlovitch, some exclamation which showed the prisoner that she was not there.
He knocked the signal the old man had agreed upon with Smerdyakov, twice slowly and then three times more quickly, the signal that meant “Grushenka is here!”
---
This knock is the one that would cause Fyodor to come to the window. Because he uses this knock, Dimitri knows Grushenka isn't there. He didn't need to hear any exclamation from Fyodor.
---- The Knocks ----
You look out for her,’ says he, ‘till midnight and later; and if she does come, you run up and knock at my door or at the window from the garden. Knock at first twice, rather gently, and then three times more quickly, then,’ says he, ‘I shall understand at once that she has come, and will open the door to you quietly.’ Another signal he gave me in case anything unexpected happens. At first, two knocks, and then, after an interval, another much louder. Then he will understand that something has happened suddenly and that I must see him, and he will open to me so that I can go and speak to him. That’s all in case Agrafena Alexandrovna can’t come herself, but sends a message. Besides, Dmitri Fyodorovitch might come, too, so I must let him know he is near. His honour is awfully afraid of Dmitri Fyodorovitch, so that even if Agrafena Alexandrovna had come and were locked in with him, and Dmitri Fyodorovitch were to turn up anywhere near at the time, I should be bound to let him know at once, knocking three times. So that the first signal of five knocks means Agrafena Alexandrovna has come, while the second signal of three knocks means ‘something important to tell you.’
