This is pretty solid real-world data, IMO — even if they’re not testing correctly to find it, it’s there in the ONS data
Testing people with any of the three ‘classic’ symptoms would have spotted 69% of symptomatic cases, with 46 people testing negative for every person testing positive. However, testing people with any of seven key symptoms – cough, fever, anosmia, fatigue, headache, sore throat and diarrhoea – in the first three days of illness would have detected 96% of symptomatic cases. In this case, for every person with the disease identified, 95 would test negative. Researchers also found users of the Symptom Study App were more likely to select headache and diarrhoea within the first three days of symptoms, and fever during the first seven days, which reflects different timings of symptoms in the disease course. Data from the ZOE app shows that 31% of people who are ill with COVID-19 don’t have any of the triad of symptoms in the early stages of the disease when most infectious.