They don't need to. "Our proprietary modelling of sea currents suggests this search area", which just happens to include the known location of the wreck.
Well, imagine if the Titanic was constantly on the move and much much smaller. And imagine if it used a nuclear reactor so that it left no debris (smoke or whatnot) behind.
And that's roughly the difficulty of finding a submarine. If anyone is seriously thinking that they can find all of an advanced nation's submarines and disabling them in a first-strike style attack... well... yeah. Its just not feasible with current technology.
