DD2 wasn't anything remarkable as a baby or toddler, infact we thought she might even be a bit slow compared to her very bright/chatty older sister.
But when she was about 3 my Mum moved to a new area, and only the 2nd time we drove there DD2 already knew the way, exactly (saying 'go right...go left' etc to me).
Still 3, her nursery told us she had an unusual affinity for numbers, and had been observed doing simple mental arithmetic in her head.
At 4 DD2 could suddenly read. It was bizarre. I certainly never taught her, and there hadn't been time for her to learn at school. She just could. At 5 she was free reading the Rainbow Fairies books. At 6 she read The Hobbit and A Christmas Carol.
Her memory is eidetic, as far as we can work out. She remembers everything.
At 9 she could finish DD1's 11+ homework faster than DD1. Despite having never seen anything like it before, and not having had any tutoring.
Her primary school never officially told us she was G&T, but they did arrange for her to attend several maths workshops for high ability children.
We sent her to the same 11+ tutor that DD1 had seen because we thought it only fair. After her first session with him, he told me 'I think we have a winner here'. He later told me he had never had a pupil so adept.
DD2 scored 277 out of a possible 280 in her 11+, and I know she finished both papers in just under half an hour (a friend was moderating and watched her).
She's now at a girls' grammar, and in the top set for maths. She's predicted virtually all 8.9s at the end of Yr 8. She rarely has to revise anything, probably because her memory is eidetic.
Interestingly, she doesn't need much sleep. Even more interestingly the grammar have also never officially told us she is G&T either (although it is noted next to her name in the form register).