Why is it you hear about people who had squeaky clean colonoscopies only to have full blown colon cancer two years later?

Lynch syndrome: A majority of folks who will get colon cancer have some benign polyps first. Someone carrying of of the lynch family of mutations is likely to get colon cancer out of the blue, with no preceding polyp. You're still smart to get 'scoped every so often, and people known to the lynch's get scoped perhaps yearly.

Sometimes: There are flat lesions that can be missed on colonoscopy and there are also rapidly growing tumors to explain this.