One other note: How well can they detect us? US subs find it nearly impossible to detect another US sub.
While U.S. nukes are the quietest there are, there is a limit. Reactors require pumps, pumps produce noise, and an extremely sophisticated detection system like the Swedes have, coupled with experienced and well trained operators, can detect anything detectable.
one way is to look for holes in the ocean... your passive detection decides on what the normal ambient background noise level is, then, you look for places that are LESS than the normal background level. because SOMETHING... is between you and what should be out there and if there's nothing on your charts, good chance it's not a good thing.