best-kept secret

:  kept from knowledge or view :  hidden

:  marked by the habit of discretion :  closemouthed

:  working with hidden aims or methods :  undercover

:  something kept hidden or unexplained :  mystery

:  something kept from the knowledge of others or shared only confidentially with a few

:  a method, formula, or process used in an art or operation and divulged only to those of one's own company or craft :  trade secret