North Korea is sending balloons filled with trash and even excrement over to South Korea, but recently something more alarming has been found: some of these balloons are now carrying GPS transmitters. South Korea’s military has discovered this new development, which suggests North Korea might be using the balloons not just for creating a nuisance but also for spying or testing new technologies.

For months, North Korea has been launching waves of these balloons, which usually burst in the...