Was it not possible to see Illumi's needle with gyo? Hisoka uses In to conceal his bungee gum which is visible with gyo, so I figured Illumi must have also masked his needle with In. I think anyone could see with gyo that Killua has a different aura on him, even a little. Because that is certainly a nen needle. 

So is Killua's luck just so bad that nobody ever noticed it and said something?
well first of all killua didn't know how to use nen so of course he couldn't see what illumi was doing
And second of all I dont think anyone suspected anything while Illumi was putting a needle in killua's head