Oh imagine the scenario of the Three Brothers where Severus is Death.
He knew Harry had to die, but either because he respected every promise now and fate granted him the last promise (to Lily ie "Life"?), or because he made a pact—he became Death.
His body couldn’t be found, and his soul is not in the afterlife per say, as they serve together to become more of a traveler between worlds. He gave his being into the purpose of helping people reach the other world.
Thanks to his power—or precisely because he sold his being for that—he ensured Harry would die but then ressucitate. He also ensured Albus, his mentor, had a talk with Harry, although Albus may not know it; and that Voldemort was well captured by death’s hands—his own hands.
I really like the idea that his body wasn’t found in the Shack because he became some sort of "superior" being, the concept and the incarnation of Death (a bit like Madoka as a goddess).
