"We don't want to overburden our digital menu boards by putting everything on there because simplicity and ease of ordering is probably the most important thing in this business," said Chris Ponzio, director of marketing at BurgerFi, a Florida-based all-natural burger chain. "People do not like to stand in line and say, 'Oh, my God, I'm overwhelmed.'"
The secret of BurgerFi's hidden menu has been intentionally spilled since its first store opened in 2011. The company even features the off-menu items on its in-store menu handouts—but not on the big board itself.
Some of the items began as a way to simplify the complicated ordering process with picky customers. Steve Lieber, the company's brand ambassador and franchise sales director, recalled the origins of the Hippie Veggie, a sandwich first created by a frequent diner in Florida.
"He wanted instead of one vegetable patty, he wanted two," Lieber said. "He didn't want them fried—he wanted them grilled. He didn't like the wheat bun—he wanted a regular bun. He didn't like the lettuce, tomato secret sauce. He liked green neon relish."
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To help employees ring up the item, the company added it to the secret menu and its internal payment system. Now, the custom sandwich is selling by the hundreds. The menu also features items created by employees and serves as a sort of soft landing space for one unpopular dish that used to be on the regular board.
The not-so-secret menu was first pioneered on a large scale by fast-food chain In-N-Out. Carl Van Fleet, the company's vice president of planning and development, said the company doesn't see itself as having a "secret menu" at all but rather just a willingness to fulfill orders just the way the customer wants it. Several items from its not-so-secret menu, including the 4x4, a cheeseburger with four beef patties and four cheese slices, have inspired fervent followings.
"Over the years, many of those variations were given names, usually by the customers who frequently ordered their burger that way," Van Fleet said. "We never set out to create or pioneer a 'secret menu,' some of the names for those variations just stuck."
Other restaurants, such as Panera, use the menus to cater to diners following a specific diet, such as a low-carbohydrate or low-gluten one.
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"Panera didn't want to take up valuable menu board space with items that only had limited appeal, but they did want customers seeking low-carb items to know that they were available," said Bret Thorn, senior food editor at Nation's Restaurants News, a trade publication.
Chris Hollander, Panera's vice president of marketing, said offering "off-menu" items is a way to serve the needs of niche groups, while also keeping its in-store messaging streamlined and consistent.
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"By keeping this menu 'hidden,' we can speak to this audience without investing in the infrastructure needed to promote these items within our walls," he said.
The program was also a way to reward its loyalty program members and social media followers, who were the first to learn of the new items.
"We are constantly looking for new and innovative rewards—beyond free menu items—in order to build deeper relationships with our MyPanera membership," he added.
These menus also give consumers a sense of insider knowledge and another way to experience a restaurant after they've already tested out the main menu, said Sam Oches, the editor of QSR Magazine, a separate trade publication.
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"Customers just really enjoy being able to go somewhere and to feel like they're on the inside, like they're in the know," Oches said.
That insider feeling often spurs customers to share their orders via social media, leading some to spread virally.
"Secret menus have been around for decades in some places, like In-N-Out," Thorn said. "And Starbucks has offered things like short cappuccinos probably since the chain opened. But social media has really made them catch on."
And as these items proliferate, restaurateurs have an added incentive for keeping these items hidden. Many of the secret items are made by combining multiple menu items. Take McDonald's Mc10:35, it combines a McDouble and an Egg McMuffin.
But sometimes these duos, can pack quite the waistline punch. By keeping these items off menu, their calorie counts stay out of sight as well—good news for fans of the quesarito, or a cheese quesadilla wrapped around a burrito, which can set customers back more than 1,000 calories.
-By CNBC's Katie Little; Follow her @katie_little_
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