Jack in the Box Wiki

Jack's Secret Sauce, formerly just called "secret sauce," is a condiment used in some Jack in the Box food items, most notably the Bonus Jack. It was also in the Jumbo Jack until May 1999, when it was replaced by ketchup.[1]

A 1983 San Diego Union article about company founder Robert O. Peterson may have given away the secret:[2]

A prime mover in San Diego affairs, he's a publicity-shy maverick outsider, a Republican who supports Democratic politicians, a lover of art whose trademark was a cartoon figure of a puppet popping out of a box, a gourmet who poured mayonnaise, mustard and chili sauce into one pot and called the result "secret sauce." 

In the Jack in the Box Universe, Jack's Secret Sauce is a highly sought-after commodity even though it's "kinda obvious" what it is.[3]