Re: Different kind of private lines?
I have used them before to reveal a detail that might have seemed normal and may be just be unnecessary details that only one or more person notes. Things that could be mistaken for mundane. Since the aren't supposed to get any sort of indication it is a secret. Say someone smells and can identify the stench of brimstone... Or some other smell that is obvious to someone who recognizes it, but may just be smelly to others. That way you can let people know, but not draw extra attention to the detail.
Were you a recipient and you got an indication by an orange underline? Or were you a co-GM when you saw it?