The night of the fire, during the coldest winter since 1885, Atticus wakes Scout up at one in the morning. He tells her to put her bathrobe on and her coat over that. She sees fire rolling out of her neighbor Miss Maudie's windows. Because the wind is blowing sparks in the direction of their home, Atticus tells Jem and Scout to stand in front of the Radley place and to stay there so they'll be out of harm's way. They watch the activity as the house burns and the neighbors try to prevent neighboring homes from catching fire. Scout and Jem talk about what's going on, and Jem continues to assure Scout that it's not time to worry yet.
Eventually Scout realizes how cold she is, that she is "slowly freezing where I stood." Jem tries to warm her with his arm, but she shakes free and dances a little to warm herself. The children keep watching until dawn when the fire is out and all the neighbors leave. They cross the street, and Atticus leads them home. As he is preparing hot chocolate for them, Atticus notices a blanket around Scout's shoulders and asks whose it is. Scout looks down and for the first time realizes she is "clutching a brown woolen blanket" that is draped around her shoulders "squaw-fashion."
When Jem starts to explain that they had stayed in place, he realizes that since they were standing with their backs to the Radleys' gate, and since Mr. Nathan Radley was helping put out the fire, the only one who could have put the blanket around Scout was Boo Radley. Atticus understands and is about to suggest returning the blanket, but Jem objects, not wanting to cause trouble for Boo. Atticus says they'll keep all the goings-on between Boo and the children—and the blanket—to themselves. Only then does Scout realize that Boo was the one who placed the blanket on her shoulders. Jem play-acts Boo sneaking up and putting the blanket around Scout's shoulders, which makes Scout queasy. Atticus later tells Miss Maudie about Boo's kindness, and she comments to Scout about the "unexpected company" she had. She says she wishes she had been there—that she would have turned around.
So although Jem, Scout, and even Miss Maudie would have loved to see Boo sneaking out of his house to perform his act of kindness toward Scout, no one actually observed him.