I vaguely remember seeing the video where Jim talks about what we (the readers) would pick up on when we eventually reread the entire series. The way I remember it, Jim only used Mouse and the Snoopy doll as an example. I think there are many things we are supposed to discover on a later read through. Some are still hidden to us. For example, there could be something about Mab or Lea that will be revealed much later on, but when we reread Grave Peril, Summer Knight or Proven Guilty after all has been revealed, there will something that makes us say to ourselves, "So that's what was this bit of action or dialog was really hinting at."
I haven't done a reread or listened to the series (so far) in a couple of years. So I can't think of anything specific off hand, so I'm wondering what other plot elements or character developments do you remember being foreshadowed in an early book that eventually became clear later on?