This happened to my younger brother about a month ago...
My brother was sat on his couch in his attic flat with his partner relaxing watching TV. Then suddenly they
both heard a knock on their flat door. They both got up to investigate and literally 3 seconds later a power cut hit. The whole area was plunged into darkness.
Anyway, they managed to get to the door but
nobody was there! They even peered down the hallway and looked down the stairs but there was not a soul around.
So was it a phantom bang on the flat door that was alerting them to an imminent blackout?! Why? What's the point? There's been other power cuts in that building before but never a knock just before one occurs. It's a head scratcher.
You see, we all get quite a few of these types of incidents but we never get any definitive answers. It's just frustrating every time. No progress gets made.