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 five years ago none of these agents were ever 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, or activists trying to aid in the overthrow of the North Korean government. Now the southerners have to fear defectors living in South Korea will be recruited for spying and assassination work.
The first of the assassins from North Korea 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 or at least in isolation from other agents. This indicated a high degree of professionalism among the North Korean intelligence operatives. Won was a professional and was first 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 often 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 Korean military bases lecturing on the evils of communism.
Won's main mission in 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 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 investigators can tell, she never got anyone into bed who had access to really useful stuff. But the skill and professionalism of Won was scary to South Korean counter-intelligence officials, who knew that if there were more like Won in the south, they were probably hard to detect and producing useful intel for the north.
There are over 20,000 North Korean refugees living in South Korea, half of them in or near the capital. The number arriving each week has gone from 30 to over a 100 in the last eight years. There are over 3,000 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. There are believed to be at over 600,000 North Korean refugees in northern China, nearly all of them there illegally.
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 gotten 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 increased the screening process from 90 days to 180 and now there will be more scrutiny of defectors who passed that screening already.