In Japan, there ARE stories about “living ghosts”. They don’t see the soul as a single “unit”, but more like… you could say, like a “fluid”? So in some cases, it’s possible that part of your soul “leak” from you, and form a living ghost. Often a vengeful one, but other cases exists too, like jealousy, over protectiveness, or obsession.
basically, Michi could have been haunted by… herself… for a while, and it might even be young Michi that is the source of many problems, even Aisawa’s death… Maybe she was obsessed with Aizawa and her living ghost killed her to bind her to Michi… A lot of different possibilities! O-O
But yeah. In Japanese folklore, you don’t need to die to produce a ghost.