My brother forwarded to me a letter that had been sent to him (he lives in France), from Nationwide. It was re an account held by our father (who died in 1991, over 30 years previously). We both suspected it might be a scam but I took it into Nationwide and amazingly, it was genuine! I was really surprised. They had been going through dormant accounts, trying to find the holder or heirs if deceased.
I had been tempted to just tear it up.
My Mum inherited everything when Dad died, but I think his Nationwide account was his "secret stash" (although I had known he had one) and it got missed. My siblings and I got a payout of nearly £30 each (after 30 years + of interest).