I've been thinking about it and I can't fathom how we would know of every case.
What percentage of adults who get the seasonal flu go see a doctor and get diagnosed, 10%? Coronavirus symptoms mimic the flu and can even by asymptomatic in people, meaning people can easily attribute symptoms to a bad cold/flu or not even notice them. On average someone with coronavirus spreads it to 3-4 other people. It's estimated that the virus started infecting people in early January, but China didn't start quarantining people until 3 weeks later or so.
I just don't see the math adding up, all it takes is a handful of people not seeking treatment in order for it to start spreading without our knowledge. There has to be thousands of additional cases around the world of that just aren't diagnosed, which is how we're suddenly seeing unexpected diagnoses in places without any previous outbreak like Iran.