ALmost all of these are what we would call “Midget” subs. Very small, with maybe one torpedo.
They can work very close in to coastlines.
But should they stray south, into South Korean waters very far at all, they would be found. I bet the South Korean Navy, which is very squared away, is working with the US Navy right now to tag as many of them as they can.
You cannot call them “Midget Subs”. They are length challenged. :)
Seems to me that it would be wise to have something like SOSUS carpeting the bottom of their waters, with very well-guarded shore-side infrastructure.