There are several countries threatened by the Chinese navy that could take unilateral actions against their submarine fleet, resulting in Chinese submarines lost with “plausible deniability” of any hostile action.
The first indication would be when they hit crush depth.
Such incidents have happened before, such as right before its collapse, the Soviet Union lost 4 submarines in 2 months, all victims to a systemic maintenance problem. But all the devil to figure out, because there were no boats left to examine.
While these are indeed acts of war, the Chinese encouraged the North Koreans to sink the Cheonan, so they are not blameless, nor have they been dissuaded from committing such acts again.
your supposition that the Chinese nuclear threat will be defeated by covert destruction of these subs, is a fantasy.
They are building a huge first strike capability which includes mobile icbm’s in thousands of miles of tunnels.
Note also the current need to place these subs off N Korea means the Chinese will let nothing happen to N Korea.