I went for a long run this morning. Didnt come within 2m of anybody else at any point but passed a whole lot of people. I'd assume that there's a chance the Google notifications will register a car passing me with somebody covid+ in or maybe even if I ran past somebodys flat...
doubt it would relate to this morning. I assumed more like someone you were near 5 days ago has since tested positive.
also I think most people only have it on when they're out and about, not sitting at home.
that's my interpretation