I suspect that our sub boys know where every one of them is.
Certainly the South Koreans have excellent sub-detection technology (probably on par with the US as we supply most of their defense needs). Yet, five years ago, a North Korean sub sank a South Korean naval vessel (see:
http://www.bbc.com/news/10129703 ).
Diesel-electric subs are very quiet (even the older ones) and would most likely pose a real threat to any vessels in the area, unless the vessels constantly used active (pinging) detection.
