My brother (who was still living at home) and my father were out at work and my mother was out as well – possibly shopping for the meal that evening – but I knew one of them would be home shortly so I let myself in with my back door key and sat down in the living room to wait for someone to return.

I didn't have long to wait.  I heard what sounded like my brother coming in through the back door, but he seemed unusually heavy footed, as if he was deliberately stamping his feet as he walked.

I thought he was messing about and that maybe he didn't realise I was in the house.  I prepared to surprise him as he walked into the living room.
On the contrary, I was the one who was surprised.  Instead of my brother, a total stranger tramped into the room, looked around briefly and sat down in an armchair...