Tigers have been around for thousands of years. We know how they hunt, their shape, and the sounds they make. Are you going to see it if one is standing in the tall grass 100 feet from you?
There is a big difference between knowing about them and being able to detect them.
If that tiger was important enough to detect (like a sub would be), you can bet your ass that we'd be using infrared detectors for that tiger, rather than visual eyes. The tiger would stand out like a sore thumb. In fact, you'd be able to see the stripes in infrared. Do you think that our submarine detection system consists of former Olympic swimmers hanging out in the middle of the Gulf, reporting "nope, I didn't see a submarine go past me."
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On another note, is it possible that we really did know they were there and aren't admitting it, and they knew that we knew and they aren't admitting it, but perhaps some other country (China) might be interested in purchasing some of their submarine technology? For them, win - China will pay big bucks and help their economy. For us, win - China purchases "obsolete" equipment that we already have counter-measures for.