http://www.washingtonpost.com/wp-dyn/content/article/2008/04/02/AR2008040203952.html?nav=rss_nation
Prosecutors called Chi Mak the "perfect sleeper agent," though he hardly looked the part. For two decades, the bespectacled Chinese-born engineer lived quietly with his wife in a Los Angeles suburb, buying a house and holding a steady job with a U.S. defense contractor, which rewarded him with promotions and a security clearance. Colleagues remembered him as a hard worker who often took paperwork home at night.
"Taking work home at night" is often an indicator, especially if that "work" is classified...

Prosecutors called Chi Mak the "perfect sleeper agent," though he hardly looked the part. For two decades, the bespectacled Chinese-born engineer lived quietly with his wife in a Los Angeles suburb, buying a house and holding a steady job with a U.S. defense contractor, which rewarded him with promotions and a security clearance. Colleagues remembered him as a hard worker who often took paperwork home at night.
"Taking work home at night" is often an indicator, especially if that "work" is classified...