Ever read the book 'The Hunt for Red October' by Tom Clancy? Remember all the details about how submarine sonar operators can track their enemies over hundreds of miles and pick up enough clues to tell precisely which boat they are listening to? (the film is slightly less tortuous on this point but comes to the same conclusion).
Well, those submarines from the UK and France seem to have proved that this is a complete myth. The couldn't even hear another sub well enough to avoid smacking into each other. No caterpillar drives were in operation (we assume), just standard missile boats going about their daily business.
Instead of the high tech world of Clancy, it instead brings to mind that scene in 'Das Boot' when the two German U-boats find themselves within a few hundred yards of each other by mistake when they should be patrolling completely different stretches of the Atlantic.