Well, because of the Interdictor fields the closes they would get in Warp is a few systems away form our main. We would be warned and able to defend
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[spoiler]Remember warp drives bend time around them it would be different if they thrusted past these fields then you be able to detect them. but the bubble around the ship allows to pass through time in a faster speed so detection is possible but quite low[/spoiler] [url]http://www.bibliotecapleyades.net/ciencia/negativeenergy/wormimages/worm7.jpg[/url]
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I see what you're saying, but Interdictor fields don't detect, they just disrupt, and knock ships out of warp