Just like when we're flying, submarine inertial navigation accuracy depends on many factors such as the length of time since the sub had a positive fix on an external reference (whether visual - not easy in that place- or sattelite or radionav). However accurate inertail plant may be, there always comes a time when the estimated position has been drifting a bit. My guess is, would that sub have detected some signal, they would have soon fixed their position, but at that time, the inacurracy was not a big deal as long they had nothing to report.