A colleague called me yesterday. He went to an asymptomatic testing centre in the local park, to do his bit as he said, not thinking in the slightest that he might be infected. Test was positive to his huge surprise, and has just been confirmed as positive by the follow-up pcr test!
He has been, like me, back on furlough since before Christmas, lives alone, goes for a bike ride once a day, has the occasional chat with neighbours across the garden fence.
First case I have heard of first-hand where the source of the infection is a complete mystery.