Secret menus aren't so secret anymore. This isn't on par with learning Bigfoot exists or confirming Area 51 houses unidentified flying objects, but in the past few weeks, it's become more widely known that some restaurant chains have secret menu items. These are food and drink items any customer can order, provided the customer knows to ask for it. "In-N-Out Burger has been doing it for at least 20 years," says Jonathan Marek, senior vice president of Applied Predictive Technologies, a business analytics software company based in Arlington, Va., that works with some of the largest restaurant chains in the world, including Panera Bread, Subway and Wendy's. Marek says secret menu items have gained prominence particularly in the past five…