Earlier this year, South Korean police arrested two North Korea agents who had entered the country pretending to be refugees from North Korea. The two were on an assassination mission. For decades, South Korea believed that North Korea would try to sneak spies into South Korea by having agents pretend to be refugees. But until two years ago, none of these agents was never caught. Most of those caught since then appear to have been sent to locate and kill refugees who were officials in the North Korean government, and were a constant source of embarrassment to North Korea because of all that inside knowledge.
The first of these assassins was discovered back in 2008. A 35 year old North Korean woman, Jong Hwa Won was arrested after being observed by South Korean intelligence for three years. The South Koreans were hoping Won would lead them to other North Korean spies, but she appeared to be operating alone.
Won was a professional, and was sent to northern China a decade ago to help the Chinese identify North Korean refugees (who were then sent back to North Korea, where they were punished, and sometimes killed.) Won had a secondary mission, to arrange the kidnapping of South Korean businessmen, and transporting them to North Korea (for what purpose is unknown, apparently even Won did not know). The kidnapping mission was cancelled before it could be carried out, and Won was ordered to get into South Korea as a refugee from the north. She did this in 2001 by the simple expedient of marrying a South Korean man doing business in China. As soon as Won got to South Korea, she divorced her husband, and offered her services to the South Korean army as a lecturer on conditions in North Korea. Won is apparently quite convincing in whatever she does, and she was soon going around to South Korea military bases lecturing on the evils of communism.
Won's main mission South Korea was to locate high ranking North Korean defectors living in the south, and kill them. She was never able to make much progress in that area. She was able to collect a lot of low level intel on the South Korean military. She did this by getting friendly with South Korean officers and used sexual relationships to get obtain classified information, especially anything on high level North Korean defectors. This is apparently how she was found out, but at least one officer, a captain nine years younger than Won, continued passing along classified info even after he figured out she was a North Korean spy.
Won would travel to China to pass information to North Korean intelligence officials, who would carry it back to North Korea. As far as the South Korean can tell, she never got anyone into bed who had access to really useful stuff.
There are over 18,000 North Korean refugees living in South Korea, and the number arriving each week has gone from 30 to nearly a 100 in the last five years. There are over 2,000 North Koreans who have obtained asylum in other countries. Many more are getting out of North Korea, but it's difficult to get from China to South Korea. This is usually done by travelling across China to a Southeast Asian nation, like Thailand, and asking for political asylum there. That usually results in the South Korean government stepping in and transporting the North Korean refugees to South Korea. China does not want to encourage North Koreans to sneak into China, by making it easy to get to South Korea from China.
There are believed to be at over 400,000 North Korean refugees in northern China, nearly all of them there illegally. A survey of these revealed that 40 percent of them had never encountered any foreign food aid, and that nearly all of them left North Korea because of food shortages.
Attracted by the opportunity to settle in prosperous South Korea, many of the millions of Chinese, of Korean ancestry, try to sneak in as North Korea refugees. There are schools in northern China that will train these Chinese, who already speak Korean (with a North China accent) to sound and act like North Koreans, and pass the intense interrogation South Korean intelligence officials give each refugee who makes it to South Korea. Most of these Chinese appear to succeed. That is worrisome, as North Korea also runs training programs to help their agents get past the South Korean security screening. Thus it is likely that many North Korean agents have got past the screening and are operating. But are they? It's just as likely that many of these agents realized, once in South Korea, that all the North Korean propaganda about South Korea and the West was a lie. At that point, all they have to do is go dark and hope for the best, or become double agents for South Korea. Perhaps in light of all this, South Korean intelligence has recently increased the screening process from 90 days, to 180.