Why do Nancy Ewalt and her father suspect that something is not right at the Clutters' house? (Page number too please!)

Nancy rang the doorbell at 9:00 am as she usually does. Nobody answered. She rang again but nobody came. This was most irregular because this family was usually prompt with answering the door. Nancy could see both cars parked in the garage so she knew they had not yet left. She turned round toward her father and shrugged at this most unexpected situation. I don't have my book, but I'm pretty sure it's in chapter 1.