Contract tracing is more likely to identify sources of infection when the people remember being in the same room with someone who had symptoms. This is also why public transport is virtually absent as a infection source in contact tracing data, but private parties are very prominent.

From what I've heard, virus concentration in the upper respiratory tract (i.e. where the aerosols come from) peaks two or three days before symptoms start.

Here in Germany there's the statistic of "75% of infection sources are not found by tracing" circulating hard through public discussions. I doubt that many of those untraceable infections could be coming from symptomatic carriers, given the level of awareness. Coughing people basically do not exist in the 2020 public. And tracing should still be good enough to recognize most cases when the source became symptomatic after the contact (which would very likely be before the receiver became aware of the infection, triggering tracing). I consider this a pretty strong indication of asymptomatic spread, or very weakly symptomatic (thresholds are very subjective).