It's easy for even a big search to miss someone, even when that person is alive. Geraldine Largay got lost on the Appalachian Trail in Maine, survived for 26 days, and died less than 2 miles from the trail, yet a large search didn't find her, and her remains weren't found for more than two years after her disappearance:
http://www.pressherald.com/2016/01/29/maine-hiker-missing-2-...