Actually, no one said anything else about that argument after the conceptual nature of souls was pointed out. If the Invisibility was something active, like Camouflage of something, I could get behind it, but it is caused by his nature as a Shinigami/soul. Considering that March can sense being far more complex than that (conceptual 1-A+ stuff), Ichigo being undetectable to her seems a big assumption to me, that kinda even borders on the NLF territory. But again, that's my opinion, people can disagree with it.