Look up Silmido in Wikipedia.

It's an island off South Korea, famous for an incident that occurred there in the 1960's. The S Koreans trained some nefarious characters on the island to assasinate Kim il Sung. The hit, however, was never done and the trained agents were quietly knocked off.

It is quite common for North and South Korean soldiers and spys to occasionally do reconosence in their counter-parts territories.

Having used to live in SK, i found it interesting that on the trains there would be advertisements telling S koreans to rat on North Korean spies if they suspected someone of being one.