Ya, theres a huge difference in not noticing someone and being invisible. To take it to almost staggering levels. I've once sat behind my sister playing my PSP(with headphones) and after like 10 minutes, she finally noticed i was there. That really is what Koishi's ability amounts to.
She could probably be at least detected by some people, though. Things like "masking your presence" is fairly common, especially for ninja-like characters, in anime and such.
Apparently, people who are big on combat rely upon some sort of psuedo-ESP to detect others, so that they can detect the life-force or electromagnetic field or soul or whatever radiating from other people, and can dodge attacks they can't even see. By closing off one's life force or soul or mind or whatever you want to call it, you can mitigate the amount of radiation you give off for people to detect, and hide even from this ESP...
But really highly trained combatants can still detect even minute traces of this, or maybe they can just apparently do something like echolocation, and can detect people by the "void" their lack of presence produces.
I'd expect grand masters of combat with centuries of experience like Youki or Yuuka to be capable of detecting this sort of trick, as well as the higher-level "strategist" type characters - Yuyuko could detect Suika when she was a mist, and set out to disturb Suika's plans, even if she didn't know what, exactly, was going on. Of course, Patchouli was able to know that Suika was someone older than Remi without any real leads, as well, and was the one who forced the start of the investigation of the geysers in SA, but she doesn't seem to be noticing now...
She got away.Hmph.I like a simple dessert like this. Everyone loves whipped cream, especially girls.Well then, let's get some tea, shall we?munchmunchWell.Don't worry about it. I got what she stole from me.It tasted like blood! They must have added it in.Let's see how this one tastes.This is great!The whipped cream is excellent! It's much better than the one I used to eat... I think...I'm satisfied.gulpWell.sip