That was a big annoyance to me too, as i really like to do the detection myself, i find the game more fun this way.
I mentionned this into this thread 2 years ago :
http://www.subsim.com/radioroom/showthread.php?t=138878
And indeed the sonar auto-crew is obviously able to use some sound isolating equipment for long range that you can't use yourself as Frame57 thought, as it can detect contact that does not appear for you anywhere on the waterfalls and other sonars.
But strangely, often the auto-crew will be unable to detect something that is obviously appearing on the waterfall and sonars.
I guess there's something wrong in the auto-crew code that was never patched.