When Jem, Scout and Dill are playing their game about the Radleys, Scout is not really into it. She tells Jem that Atticus doesn't want them to play the game anymore, but Jem and Dill still do so. Scout is reminded of what she heard when she rode the tire into the Radley front yard. She is still unnerved by all of this and just wants to stop playing, so when Atticus comes home she is glad that they have to stop so they won't be caught.
Atticus's arrival was the second reason I wanted to quit the game. The first reason happened the day I rolled into the Radley front yard. Through all the head-shaking, quelling of nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing.
What Scout doesn't realize is that Boo was watching the children play. Scout is certain of what she heard, but she is still confused and rattled by what she had heard and what it means. Little does Scout realize, she will be ever so grateful that Boo has been watching them, and she will have him to thank for their lives.