I hate to tear you away from your monkey spanking session but your the one wrong here.
In practice, one could prove that group Y cannot do action X successfully. While not a perfect from a deductive standpoint, if you have reasonable evidence that Y has been trying to the best of its ability to accomplish X, and fails, it is not inductively illogical to assume that Y cannot succeed when trying action X. The questions presented is fair, if slightly inaccurate due to the imposition of language.
In this case, we have demonstrated abilities that the Empire can and has detected small craft at long ranges - ranges exceeding by far the size of a solar system. Since Shivan vessels do not differ in composition wildly from Imperial/SW vessels, we can logically conclude they could be detected. Imperial Star Destroyer sensor technology seems to be less effective at super-close range, and this is logically not a problem since its primary function is as a heavy capital ship rather than a fighter-killer. If someone can provide good evidence that Imperial sensor technology is blind or has some specific flaw related to Shivan jamming technology, one could reasonably conlude the Shivans have a sensor advantage.
While some of my fellows here are less than polite, they were baited and insulted by a trollish poster with a chip on his shoulder.
Finally, I have played Freespace II. The Shivans [and terrans too in some ways] are arguably the least threatening space fleet I have ever seen since the Kilrathi.
1) They have "maximum velocity" in space.
2) They have very... stupid tacticas and strtategy, with no clear goals other than to kill.
3) Their capital ships are less than impressive compared to Imperial vessels. Their largest heavy ships actually are bigger than a Star Destroyer, but wastes most of the space in comparison. A full volley rom a Star Destroyer easily equals a full fleet of Shivan heavy capital ships. Moreover they seem to need to be at ludicrously close ranges to fire effectively (within a few km), although this may be due to heavy jamming.
4) I personally slew over two thousand enemy fighters, not to mention the capital ships I helped take out. My character, on average, died once for every 250 enemy. That is *not* an acceptable kill ratio. Moreover the speeds and firepower of the craft I was using do not compare to the Imperial TIE. And in flight combat, speed and firepower are almost everything.
Also, fighter are far more effective against Shivan capital ships, barring a lucky shot such as the A-Wing that, against all odds, took out the Executor. (I've always wondered if the Force didn't give a wee little bit of help there.)