I've had that several times. Why it happens, I don't know. In my freelance days, I might be off for a few days in a row and was always around for the morning post. Packages, likes SD, usually come over in a van, I think. One day, heard the letterbox flap go. Got up to have a look and there's a card on mat. 'Sorry we missed you'. etc. I ran out and caught the postie, who informed me he had no package for me and that an attempt had been made yesterday to deliver. WHich it hadn't, as I'd been in all day. Weird.