The wreckage of a crashed drone is seen on a mountain on April 6 in Samcheok, South Korea. Three drones, believed to be North Korean, have been found in South Korea in recent weeks. (South Korean Defence Ministry via Getty Images)

South Korea has long taken unusual pains to keep people from aerially photographing its presidential palace. On a mountainous hiking trail that winds behind the blue-roofed palace, cameras are restricted. The satellite mapping service produced by Naver — Korea’s version of Google — resorts to a Photoshop trick, depicting the Blue House, set just north of Seoul’s downtown, as a bushy forest.

Such covertness is often excused as the duty the of a country technically at war. Decades ago, North Korean commandos tried to raid the palace. Pyongyang could presumably plan worse.

It was with a degree of chagrin, then, that South Korea on Friday announced that the Blue House — and several other sensitive areas in the country — had been photographed by sky-blue drones almost certainly belonging to the rival North. South Korea learned about the incursion only because three such drones crashed-landed, leaving a cache of evidence. Investigators found photos of South Korean apartment blocks, military installations and President Park Geun-hye’s palace.

The drones — about six feet long, made of polycarbonate — are hardly the North’s most imposing weapons. They look like models one might buy at a hobby shop. But the South is taking them seriously. For one, they skated unnoticed past Seoul’s radar system. They're also marked with serial numbers, raising the possibility that many more have not only flown over the South, but also safely returned to the North, information in hand. Some South Korean media have raised alarms that the drones could one day be refitted for chemical or biological weapons attacks — though that would require some major technical leaps.

North Korea hasn't said that it operated the drones over South Korea, but its drone program is public knowledge. The North has shown off a larger version — apparent copies of a Raytheon MQM-107 Streaker, the Pentagon says — in military parades. Last year, leader Kim Jong Un supervised a military drill in which “super precision” drones assaulted makeshift targets, “destroying them with accuracy,” Pyongyang’s state-run news agency said.

At a news conference Friday announcing preliminary investigation results into the incursion, a South Korean Defense Ministry spokesman said the drones almost certainly belonged to the North, though there was only circumstantial evidence. Given their engine capacity and estimated speed, they couldn’t have come from other neighboring countries like Japan or China. The spokesman, Kim Min-seok, said Seoul views the drones as a “military threat” and plans to build a system to detect and target them.

Two of the drones are thought to have crash-landed in late March. Another turned up last October, discovered by a wild ginseng farmer. But he didn’t report the finding until the other ones made news. That particular drone was the only one from which photos couldn't be recovered: The farmer had wiped the camera’s memory card and taken it for his own, South Korean media said.