Jeff Rickey is a numbers guy. But three years ago, a colleague asked him about something he'd never counted: applicants who came out of nowhere. The question intrigued Mr. Rickey, dean of admissions and financial aid at Earlham College in Indiana. He found that 17 percent of the college's applicants that year had not called, taken a tour, or requested a viewbook. Since then, Earlham has seen a steady increase in "stealth applicants," the term some admissions deans use to describe students whose applications are their first contact with colleges. That jump reflects a national trend, according to a new "Chronicle" survey of admissions deans and enrollment managers. Forty-one percent of respondents said the number of stealth applicants had increased greatly at their colleges over the past 10 years, and an additional 37 percent said the number had increased somewhat. The uptick was more pronounced at private colleges than at public ones. This article describes how out-of-the-blue applications complicate the jobs of admissions officials, who traditionally have known at least something about applicants long before their applications arrive. Tracking students' visits and requests for information allows colleges to predict how many applications they will receive each year. Applicants who act as "secret shoppers" until they finally apply, however, warp those estimates, as well as the formulas colleges use to estimate their all-important yield rate (the proportion of accepted students who choose to attend).