It is in Chapter Eighteen of this great book that you need to refer to. Understand that Chapter Seventeen narrates the abortive escape attempt that was planned. Hannah sees "shadows" before she returns with Gitl to their bunkers, but it is only during the next day, when the people that tried to escape are brutally slaughtered in front of the Jews, that Hannah realises that Yitzchak was not there among them. There is therefore hope that at least one of their number escaped successfully and has not been detected by the Nazis. When Hannah points this out to Gitl, she responses with a voice that "held a measure of hope." This makes Hannah in turn smile with this hope:
Hannah said no more, but in her mind's eye she saw a swift shadow racing into the dark trees. She smiled with the memory.
The escape of Yitzchak offers hope to those remaining, that there may be a life outside of the concentration camps and a possible future. This is a hope that is incredibly important to them in the atmosphere of death and despair in which they exist. This is why Hannah smiles at this memory of the "swift shadow" escaping, which she now realises must have been Yitzchak.