For some context, researchers in China went back and reran old flu surveillance samples from Wuhan taken in December and the start of January. According to the WHO joint mission's report, 1 out of the 20 samples from the first week of January and 3 out of the 20 samples from the second week tested positive for Covid-19. China locked down Wuhan on the 23rd. Things were probably already really bad by that point in a way that wasn't reflected in the number of confirmed cases.