This is mostly conjecture, but one of the flavors I've been able to detect in Coca-Cola is an unexpected, but logical one: Riesling, as in the wine or grapes. This has led me to the following speculation: it's a fact that Coca-Cola is ersatz Mariani Wine, a cocaine-laced beverage (think Bawls on steroids) of the late 19th century. An extract of wine grapes (even more than the cocaine) would be suspect in some Prohibitionist quarters in America (dry counties, sales to children, "Negroes", "Indians", etc.) Therefore, by claiming a "secret" ingredient, the Coca-Cola company could market their product as a "temperance" drink, with no such problems.

My opinion only, YMMV.