If the US knew of a way to locate subs, then it would worry that Russia or China would figure it out, too
There are many conceivable ways to track subs and this is only part of the problem because subs still need to be destroyed after being located. Russia and China combined don’t have enough nuclear attack subs to credibly do this to the US. The US does have enough nuclear attack subs to credibly destroy Russia’s deterrent fleet, if they can be tracked, with attack subs left over to defend our own ballistic missile subs. A primary mission for nuclear attack subs is to shadow nuclear ballistic missile subs. That Russia is (allegedly) developing weapons like Status-6 and Burvestnik suggests they are not satisfied with the ongoing deterrent capability they already have.
Also, about 2 thirds of the US’s 1357 “strategic” (capable of incinerating the heart of a major city) nuclear warheads are currently on subs, rather than in missile silos or on bombers
The number of weapons deployed, and where they are deployed simply isn’t verifiable. Keep in mind that it is widely held, and codified in public law, that use of nuclear weapons by the US including for retaliatory purposes must follow the kind of centralized authorization that could be extremely difficult to guarantee under a surprise nuclear attack. This would open us up to surprise decapitation attack so the probability it’s true in practice is very low.