The researchers seem to find it in sewage first. There are several areas of Hong Kong where the virus has been detected in sewage outflows but there are no "reported" cases in the area.
I heard that DB is one, and Tung Chung another, to keep it in an aviation context.
I believe it is so true that because catching it was made a crime in Hong Kong attracting a custodial sentence in a secure overcrowded hospital, it has made people hide their infections. In most other countries you do a home test and if positive go home to bed and stay clear of other people for a week. You don't have to be an epidemiologist to work out which is more effective at slowing an epidemic.