Secret ingredient

A secret ingredient is a component of a product that is closely guarded from public disclosure for competitive advantage. Sometimes the ingredient makes a noticeable difference in the way a product performs, looks or tastes; other times it is used for advertising puffery. Companies can go to elaborate lengths to maintain secrecy, repackaging ingredients in one location, partially mixing them in another and relabeling them for shipment to a third, and so on. Secret ingredients are normally not patented because that would result in publication, but they are protected by trade secret laws. Employees who need access to the secret are usually required to sign non-disclosure agreements.

...
Wikipedia