Scout thinks Calpurnia leads a "double life." What details lead Scout to think this?
In Ch. 12 Calpurnia takes Jem and Scout to her church, First Purchase, and it is here that the children see Calpurnia in her own home setting, in her own neighborhood, and among her peers. Prior to this it seems their only experiences with Cal have been in and around their own home.
In the Finch home, Calpurnia is quite literally "the help." She may be well loved and be practically raising the two kids with Atticus, but she is still hired help. When she takes the kids to church with her they are able to see Calpurnia as a hostess. She is the one in control and they are her guests. Their is a bit of a shift in power.
Additionally, as the previous educator noted, Calpurnia speaks differently among her peers than she does in the Finch home, and it is obvious that there is more to Calpurnia than the children have been aware. The chapter does a very good job, too, of showing the other side of Maycomb, the "colored" Maycomb, as well. Most of the book talks about how Blacks are treated and how they act and so on, and this is on of the few instances in the novel that lets the reader see a bit of their world rather than them in the all-powerful and controlling white-world.
Scout realises this when one Sunday Calpurnia takes the children to visit her own church, in the heart of the black community. It is a revelation to Scout to see Calpurnia as part of this community rather than in her usual role as the Finchs' housekeeper. Furthermore, Calpurnia even speaks a different dialect in this particular world. This is really what makes Scout feel that Calpurnia leads a double life. Calpurnia has one existence in the Finch household, where white culture dominates, and quite another in the black community, where she even seems to speak a whole new language. To the young Scout, these existences appear totally separate. It is true that this is a child's perspective, but it also illustrates the nature of racial segregation in such a town as Maycomb.