Chi Mak exploited his low-key lifestyle and his good reputation to hide his real intent: conspiring to pass U.S. secrets to China for over two decades. Mak is a naturalized U.S. citizen who confessed to the crime and revealed that he lied to investigators.
This individual apparently had no warning signs that he was suspect until he finally was found out and revealed his real intent on his own.
How you combat a clever individual like this without infringing upon individual liberties is an open question.