"Nobody noticed" IMHO mainly comes from two things:
a) At least part of ones relatives are autistic themselves. So a lot of stuff might be classified as "normal" because "I did the same thing when I was a child".
b) Neurotypical people hiding/ignoring/forgetting any clues because "it is mean to tell someone that they are not 'normal'". Because NT humans love being average so much. I wonder why this isn't used more often as a compliment...