The swedes have the quietest most stealthy sub in the world. It kicks our asses EVERY time they go head to head with us. We can not find it. Not a fluke, a huge jump in sub warfare.
You and I wouldn't know about it if we COULD detect it. ASW is like modern aerial warfare -- it's all about gaining an intel advantage so that you can kick butt from the first day of the war. You don't shoot your mouth off about having tricked the enemy into turning on his radars so that you could record them; neither do you tell the Swedes that you are gaining the ability to detect their latest toy. You keep that stuff classified.
That said, this Swedish sub and AIP subs like it ARE a problem.