Actually, this is not so clear. The evidence of community cases in November/December in e.g. France were not picked up at the time either. There is a phase were a new disease can fly completely under the radar, because we are still in a very slow growth and only see a few isolated cases, which often don't even get seen by the same health professionals. This is particularly true in the case of covid were there is evidence that spreading is disproportionately from "superspreaders" (unless some of that information changed now?). So unless there was a superspreader, you only have few isolated cases.