The PM seems to have spawned the phrase on the 6th. And it has been run with. The later the most important.
Some of this is to be expected as we see close contacts and family members test positive, but some of these are still what we call unlinked or mystery cases. These matter because so long as we don’t know where a case has come from, we cannot tell if we have other chains of transmission in the community that we haven’t isolated yet. That’s a cause for concern
But, as Dr Bloomfield said, what we’ve asked for is basically to date-stamp those mystery cases, because if they were very, very early on, then, look, we accept that there were a large number of exposure events early on. But if you’re getting them later on with infection onset later on, then they are more risky to us.
I wonder how they handle infected people who are "known to the police", and have neither the will, nor motivation, to help with contact tracing.