But an investigation team sent to the area found that the object was actually the broken door of a portable toilet, a defence ministry spokesman said.
The door was a similar sky blue colour to three drones recovered by South Korea in March and April.
“The shape from a distance and the colour was probably what created the confusion,” the spokesman told AFP.
Seoul said last week that a joint investigation with US experts had provided “smoking gun” proof that the three recovered drones were flown from North Korea and had been pre-programmed to fly over South Korean military installations.
North Korea angrily denied any link with the drones, accusing the South of “fabricating” the evidence.