I don't just mean a secret ingredient or technique -- some huge percent of recipes claim that something secret about them is for the first time being revealed. And I don't mean the repeating urban legend about the authentic recipe for Mrs.Fields cookies or some recipe from a hotel in New York or more recently, a recipe from Neiman-Marcus (this link to Snopes explodes a whole list of such legends going back 50 years). These were supposedly bought accidentally and then in revenge for the high price disseminated in chain letters (before the internet) and in email chain letters (from the dawn of the internet).
I know that many newspaper food sections and magazines (such as the LA Times and the late, great Gourmet) often obtain and publish restaurant recipes, sometimes with warnings that the necessary adjustment of for home kitchens may produce somewhat different results. Obviously, the publication of a recipe may give some help to a competing restaurant. But I really wonder if a home cook (or even Nero Wolfe's private chef) could truly duplicate the flavor one experienced in a restaurant. Or if the pleasure of eating a dish in a restaurant is irreproducible -- after all the perceived taste may be affected by candle-lit atmosphere, attentiveness of the server, beautiful arrangements of food on the plate, and even the expectations created by special cutlery, china plates, and whatever else contributes to the totality of eating the dish.
The other meaning of secret recipes of course involves commercial products such as Coke or Twinkies. Lots of recipe writers have claimed the ability to reproduce Twinkies -- I don't know why you wouldn't be satisfied to buy Twinkies when you wanted them, but that's not important now. I googled "secret recipe" and found offers of all sorts of books and websites with recipes for commercial products (love them or hate them). I've never seen a home recipe for Coke or Diet Coke, which seems realistic, but there are a huge number of others. As with Twinkies, I really wonder why anyone would bother if you can so easily buy it at Kroger's, 7-11, or anywhere.
"Strange News: The Truth Behind Secret Recipes in Coke, KFC, Etc." by Benjamin Radford is an interesting article I found on the website Live Science. He implies that the idea of a "secret recipe" is mainly an advertising trick. He says: "Coca-cola has one of the most famous secret recipes in the world; ads whimsically claim that only two men know the ingredient list, and describe the dire consequences that would befall the planet if the secret was ever lost, including a hole appearing in the fabric of the universe."
Radford asks this important question: are there really any secrets in this day of sophisticated laboratory analysis and legal requirements for a list that covers all ingredients? He says there may have been real secrets in the past --
As for the secret of Coke, he says even that is on page 43 of Poundstone's book. I don't know what made me curious about this topic, but here it is.
But these days, any laboratory worth its sodium chloride can tell pretty much what chemicals and ingredients appear in what quantities of a given sample. It's food science, not rocket science.
In his book "Big Secrets," William Poundstone revealed a laboratory analysis of Kentucky Fried Chicken: "The sample of coating mix was found to contain four and only four ingredients: flour, salt, monosodium glutamate, and black pepper. There were no eleven herbs and spices — no herbs at all in fact... Nothing was found in the sample that couldn't be identified." So much for the "secret." In fact, the chicken's ingredient statement is available on KFC's Web site.