I do not believe Outsider-taint is that easily detected; not even via soulgaze.
The Gatekeeper -- likely the preeminent wizard in the field -- cannot reliably detect it.
If it were all that easy... there are plenty of Wardens. Just get a new Soulgaze on a suspect, and bam you found the taint.
And Nemesis is even harder than other Outsiders: Titania and Mab cannot detect it, only infer it after the fact. (I'm unclear that taint/possession is even all that common a trick amongst the Outsider crowd; do we have any other instances that we know are not Nemesis?)
What are you suggesting? That the taint of an Outsider is on Harry and that is what the addict saw? That Eb couldn't see it in a soul gaze, nor the Loa, nor Rashid, who I believed did his own scan of Harry with his eye? Even if he saw the spiritual scar or taint, did it really mean anything? Does it mean that Harry is possessed in any way? Or does it mean something else?
Let's check out what the exact quote is from Storm Front. page 134-135
Then something strange happened.
The young man looked up at me, and his eyes rounded and dilated, until I thought they had turned into huge black coins dotted onto his blooThey dshot eyeballs. His eyes rolled back into his head until he could hardly have been able to see, and he started to shout in a clarion voice. "Wizard!" I see you! I see you,wizard! I see the things that follow, those who walk before and He Who Walks Behind! They come, they come for you!"
The junkie said he saw things
that follow, not something that is part of Harry.
Then on page 136 Harry says he had never met the junkie, and he didn't sense any magical practitioner. So how paraphrasing, "in the hell had he seen the shadow of He Who Walks Behind in Harry's wake?" Harry goes on to admit that he is marked, indelibly with the remnants of the hunter-spirit known as He Who Walks Behind, though the Outsider didn't succeed in what it was trying to do. Then he says those who know how, using the Third Eye would spot the sort of spiritual scar left from the encounter. But this was an addict, not trained to either spot the scar or taint, nor to recognize it if he saw it.
Then Harry says; page 136
only a wizard had that kind of vision, the ability to sense the auras and manifestations of magical phenomena. And that junkie had been no wizard.
That last is the important bit,
that junkie had been no wizard. Didn't know Harry, or at least Harry hadn't seen him before, yet the junkie was calling Harry, wizard. Yeah, maybe he looked Harry up in the phone book, but still the junkie had to know what made a wizard to call Harry, a wizard! Let alone what an Outsider was, by name even! Harry goes on to say on the next page that the Third Eye allows wizards to see things supernaturally invisible or other things as they really are. Examples of that are when Harry made the mistake of looking at the Skin Walker with his third eye, the horror of that encounter turned him into a fetal ball, or when he saw Murphy with it and saw an angel. My point? Because he is a trained wizard, Harry knew what he was looking at.. He also said that wizards learn to be very careful with that vision because it can harm them.. As Mac told him only recently, the real vision of who he is would kill or drive Harry mad.
Yet this addict knew all of the stuff he told Harry! The important point isn't that Harry bears some spiritual scar from his encounter with HWWB as a sixteen year old, call it a taint if you want, its of no importance.. What is important is who is trying to train up and teach these addicts? This happened with Sells, ordinary man with a little talent that exploited, turned him into a sorcerer.. He was also one of those making and pushing the Third Eye drug. To what purpose?
They come, they come for you!
In other words it is a bit of a warning to Harry, the Outsiders were still after him.