I saw that too. It resonated with me as I had had a problem with the postie popping a "you were out" card through the letterbox when I was standing right behind it! The doorbell had not been rung. I opened the door and chased him but he made a sharp exit. He wasn't carrying any parcel. I complained to Royal Mail (who were not surprised -oddly) and to M&S who were using Royal Mail as their carrier. M&S gave me an ex gratia payment even although it wasn't their fault. Royal Mail said they would deliver the parcel the next day. They didn't, and I had to drive to the depot to collect it
Looks like these scammers are taking advantage of Royal Mail behaving like this.