they hide underwater, it can't be seen. I would imagine China have some sort of undersea tunnels for subs connecting with some land facilities. Lots of nations have the capability to build undersea tunnels now like the undersea highway tunnels in Norway. It is couple hundred feet deep under the sea. The Hanian Island tunnels have been built couple decades ago. It is old. They assumed no ones can see from above. Time has changed. They must have built couples undersea that can't be detected from above by now.

But the trend is with much smllar subs. It is much harder to detect when it resurface. It can be sort of drone that dormant deep under the ocean to preserve energy and activate when signals are received.