Washington and secrets go together hand in glove. Or maybe it’s more like cloak and dagger. It should be no surprise then that the D.C. area is one of the hot spots energizing the nation’s growing demand for secret doors — panels, bookcases, mirrors or artwork — that swing open to reveal a passage to another room. The obvious purpose of a hidden door is for security — to conceal a safe room or valuables. But as pre-built, ready-to-install doors become more widely available, people are adding them for aesthetics, for fun or maybe because they watched too much “Scooby-Doo.” For D.C. resident Nicole Buell, a bookcase that concealed a doorway solved a design problem. In her 540-square-foot Columbia Heights condo, the doors to the only bathroom were in her bedroom and the living area. The living area door left too little room for pictures or bookcases. “It just wasn’t a good use of space,” Buell said. Walling over the door was an option, “but,” she said, “I didn’t want guests to have to go through the bedroom to get to the bathroom.” The solution began with door hinges bought from Secret Doorways, a company in Sunbury, Ohio, owned by a cousin. With the help of her father, she constru...