[stordoff]

Almost certainly[1], my grandad caught it in hospital, then infected most of the rest of the family (6/7 people) when he was discharged.

He was admitted with suspected sepsis, and tested negative on admission. He had a new cough when he was discharged, but we didn't think much of it as he had COPD so periods of coughing weren't unusual. A few days later, he was readmitted with a recurrence of his original symptoms, but this time his admission test was positive. Six of the other family members tested positive the next day, and developed symptoms over the following days.

[1] The only other person in the house who hadn't been isolating subsequently tested negative. Everyone else had been nowhere but home for at least the proceeding two weeks, so the chance of another vector seems remote.