The app appears to be sub-optimal
https://news.sky.com/story/coronavirus-some-users-of-nhs-tracing-app-incorrectly-given-covid-19-exposure-alerts-12086225
That looks like what it was. Given I've not left the house in the past 3 days it seemed a bit strange.
I'd expect a lag of 5+ days for a notification. You need to be in contact with someone that has been infected but doesn't yet know. They then have to develop symptoms, get a test and get the result which is going to take a few days.
@aggi started
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The app appears to be sub-optimal
https://news.sky.com/story/coronavirus-some-users-of-nhs-tracing-app-incorrectly-given-covid-19-exposure-alerts-12086225