What did the contents of the envelope reveal in To Kill a Mockingbird?

The only envelope I'm aware of in the novel is the one Calpurnia interrupts court proceedings to deliver to Atticus. She comes up the middle aisle, waits for Judge Taylor to acknowledge her, then reassures him that she needs to pass the envelope to Atticus and "it hasn't got anything to do with--with the trial." It is a message from Aunt Alexandra that says Atticus's children "are missing, haven't turned up since noon." 

Atticus is...

The only envelope I'm aware of in the novel is the one Calpurnia interrupts court proceedings to deliver to Atticus. She comes up the middle aisle, waits for Judge Taylor to acknowledge her, then reassures him that she needs to pass the envelope to Atticus and "it hasn't got anything to do with--with the trial." It is a message from Aunt Alexandra that says Atticus's children "are missing, haven't turned up since noon." 

Atticus is about to ask the judge for a recess when Mr. Underwood, the owner and editor of the Tribune, says, "I know where they are, Atticus. They're right up yonder in the colored balcony--been there since precisely one-eighteen P.M." 

This is a big deal, not just because they'd not told Calpurnia or Aunt Alexandra where they were going (of course, because neither of the ladies would have let them go), but because they were sitting in the colored balcony. The horror!