https://www.bbc.com/news/uk-england-kent-67653335
The postcard was sent from Sydney in August 1981 to a house in Westgate, near Margate.
it is likely that it was put back into the postal system by someone recently, rather than it being lost or stuck somewhere.
No, I think you're just saying that to save face and not be so embarrassed that the Royal Mail lost this post card in their system for 40 years. It is way more believable that the post workers lost it for 40 years than someone re-sending this 40 year old post card.
This reminds me of stuff that happened at my current workplace.
When removing an old dead printer around five years ago, we also had to move the old cabinet of staff mailboxes out of the way to get the printer out of the printer room. We found an inter-office mail envelope with official documents from March 1997, which was stuck between the (painted) wall and the cabinet.
As it turns out, the person it was sent to had just (five years ago) come back from his retirement briefly and worked as a consultant for a couple of months (for near-exorbitant fees, but he was hard to replace). Long story short, the document in question was pretty important in the 90's, and apparently led to a temporary political fallout because it was never properly approved, signed and returned.
The sender had since passed. We left a bouquet wrapped in the document (after scanning it) as a remembrance of and credit to her work and importance for our local municipality.
Similar thing happened back in the 80's at a Sorting Office where I worked. The letters, then, were sorted into a 48 box fitting. They took a row of 48 box fittings down and discovered a postcard that was alleged to be 100 years old. I think they had a little ceremony with the living descendants of the addressee. This was when the Royal Mail prided itself on delivering the mail.