There is a reported case from China of a Chinese tourist who visited Bali on Jan 22, then returned to Shanghai on the 28th, then began showing symptoms 8 days after returning to China. (Jakarta Post article).
He had flown from Wuhan - so probably already had the infection before he travelled.
If the article is correct (big ?) then he wasn't diagnosed in Shanghai but in Huainan, he probably wasn't able to return to Hubei/Wuhan. With incubation period of 2-10 days or 2-14 days (WHO and US CDC) or possibly 2-24 days (preview paper from China awaiting peer review), then that gives an idea of how complex contact tracing etc for just one confirmed case can be, and once there are many confirmed cases and growing, then the task becomes effectively impossible.
I met 10 people who met 10 people who met 10 people and so on.
People are wearing masks so they don't catch it, perhaps the tactic is to treat every person as a carrier, so that the mask and other hygiene practices in China are there to keep a potential infection within that person. Not to allow cinemas to open etc obviously, but to enforce everyone to wear a mask for protection outwards and inwards. Break the cycle. Possibly gloves as well, and no face touching. Are disinfectants any use? If a person had it, and got a little sick, then recovered, all at home, are they still a carrier?