I was so *close*...
What kills me most about this target is that I very clearly had the Silent Assassin solution thought out, but I just missed a few details.
At the end of the mission, you might notice that I didn't just lose the bonus for "No Evidence", but I also lost the bonus for "Never Spotted". I've been over this recording several times now, to create my commentary, edit the video, and now to make this post, and I never saw a point where I got detected during my "canonical" run.
This leads me to one conclusion, the only person who could have seen me... was the target. I strongly suspect that since he was turning to face me as I injected the Modern Lethal Syringe into his flesh, he saw he perform the deed. This might have been what triggered the point loss from never getting spotted.
If that is indeed the case, I would find it quite interesting that the system doesn't factor in whether or not the person who saw detected Agent 47 was still alive at the end of the mission. 90% of the time, that wouldn't matter because if the player is killing witnesses, they already lost Silent Assassin and score no longer matters. This is only important in the edge case where the people who detect the player are targets.
I don't think it's a big problem, but given that players only have one shot at Elusive Targets, any technical problem or punishing design decisions get severely amplified. It's one thing when the player can trace their failure to their own mistakes, it's quite another to lose a Silent Assassin to a technicality in these cases.