The unmanned aerial vehicle, which was discovered Friday, was equipped with a SONY digital camera that had hundreds of photographs on it, some from 2-3 kilometers (1.2-1.8 miles) high, according to an official with the South Korean military.
About 10 of them were aerial shots of the Terminal High Altitude Area Defense (THAAD) system -- more than 100 miles away from the border -- which is designed to shoot North Korea's ballistic missiles down before they can do damage, the official said.
It likely crashed after running out of fuel, the official added.
The vehicle is believed to be from Pyongyang because it bears similarities to a North Korean drone that crashed on Baengnyeong island in 2014
, according to the official, though it was slightly larger and had twin engines.
The Baengnyeong drone was found days after another one was spotted in Paju, a city that sits near the demilitarized zone which divides the two countries.
It's one of the most the heavily militarized and monitored places on the planet, and it's hard for objects to travel past it undetected.
Last month, South Korean authorities announced that an unidentified object flew across the border, sparking rumors that it might be a North Korean drone.
THAAD's deployment has been a polarizing issue for most of the region. Beyond angering the North Koreans, experts say the Chinese worry the United States could use the system's advanced radar to spy inside China.
The decision to use THAAD was made by former South Korean President Park Geun-hye, who was impeached and later ousted in a corruption scandal.
While the more hawkish elements in South Korea supported the deployment, many in the country worried about how Beijing's reaction would affect South Korean businesses and nationals living in China.
Lotte, a South Korean conglomerate which owns the golf course in Seongju where THAAD is being deployed, saw the majority of its stores in China close after agreeing to hand over the land.
Park's left-leaning successor, Moon Jae-in, campaigned on a promise to put the final decision on the missile system before the country's parliament.
Moon decided to suspend the THAAD deployment last week, pending an environmental impact assessment.