I'll take the second part first. If they have a hunt in waters off our two coasts, it comes down to a co-operative relationship with our American neighbours and the intelligence that is shared among members of the Five Eyes, a community one small part of which is that kind of intelligence.
Anything that comes to this side of the Atlantic or the Pacific—and we've had both happen in the past—would be a team effort and would involve national sensors—national-level intelligence collection methods, in other words—and national platforms at sea, of the kind the United States Navy has that are focused on acoustics. Then it would involve maritime patrol aircraft and submarines, and potentially the use of surface ships as well, in the absence of or to complement the others.
All would be involved, and not only involved, but.... You can imagine that in certain circumstances for submarines approaching North America, you would want not only to know where the vessel is, but also, if it is a cruise missile firer, to figure out what the plan is to deal with that, which, for the most part, requires being relatively close to be able to deal with it. All of the above get used.
Now, which one would I wish to use first? Maritime patrol aircraft...and then put submarines into the right position to deal with the other submarines. As always, if you can use maritime patrol aircraft alone, they have a certain invulnerability to submarines that's quite attractive.