Did some reassuring googling last night.

Apparently exposure with low risk means your phone has registered a contact with someone later tested positive, but it was less than 10 minutes or more than 2 metres away. Because they want to be "completely honest " it'll even show you an encounter that was up to 8 metres away that's not good for people with anxiety. But I do feel less anxious now I know that.
And this morning it's saying I have a low risk of infection again, and the part about avoiding close contact with people in my household has disappeared.

I feel like there's a lot of things they really didn't think through with this app though.

Anyone in other countries using an app? What are they like?