Looking back (my DD is 13 now) the milestones thing partly fit and partially didn't.
Mostly it was, as eldertree notes above, just more there there.
It was easy to ignore if you only spent time with her in a superficial manner. She was excellent at 'cloaking' by the time she was two or three. But her memory was uncanny. Not only that, she would come up with things that required: a) phenomenal recall of complex information, b) manipulation of that information, and c) expression of that information with obvious evidence of synthesis. Context-appropriate expression, I might add.
So she might use parts of a phrase from a book or television show to make a joke about something that she found funny or strange.
You know how adults who are fans of, say, The Princess Bride may use the meme "This word... I do not think it means... what you think it means"? Like that. Obviously, she hadn't read TPB or seen the movie. But she was lifting content from books and media that she HAD seen.
It was very disconcerting, because it was these very mature phrases that she wouldn't otherwise use-- she was lifting the entire phrase contextually the way some children lift single vocabulary words, for lack of a better way to explain it. It was a little freaky, though, because it was so much like echolalia. You really had to be paying attention to know what she was doing, and she never did it in front of people she wasn't comfortable with-- or, for that matter, in front of age-mates. Well, she did, a few times. With them she used more juvenile material (Barney, Mr. Rogers) but quickly gave even that up since they still didn't 'get' it.
She also recalled the 9/11 footage that she'd seen for only a few minutes on TV (live). She was just about 29 months old then, and she talked about "when the buildings fall down" and was fearful of airplanes when she was nearly three. In between, not a peep. It was as though she was processing what she'd seen and absorbing additional (adult) conversation snippets to put it together.
Looking back, those kinds of things and her extraordinary awareness of things and people around her are just so different from other children that age. Radically so. We had no idea at the time. I was having 'car conversations' with her when she was still in a rear-facing carseat. When I think about some of those conversations, they were about topics that no child under four ought to even have a concept of.
