Over the years, outside analysts have closely followed visits by North Korean leaders to factories, farms and military units to discern the regime’s policy priorities.

The sleuthing is challenging: The North Korean state news media often withhold the locations of these sites and their purposes, identifying them only by the names of their managers.

Now, two analysts based in the United States have located six such factories believed to be linked to North Korea’s missile program, visits to which by the country’s leaders were deliberately obscured by the state news media to thwart Washington’s intelligence-gathering or cyberattacks. The factories and their operations were discovered through a painstaking digital examination of open-source data.

“North Korea may be reluctant to share those locations precisely to make them harder to target,” Jeffrey Lewis, an expert on North Korea at the Middlebury Institute of International Studies in Monterey, Calif., said in a report published Thursday. “In other cases, however, the visits may have been related to the development of new missile-related systems that North Korea was not yet prepared to reveal.”