Just looked at the Multiversity guidebook; the Silver City is located in heaven along with the Pax Dei and Spectre but nothing about the Presence itself is mentioned specifically.
Maybe it is part of the Source and Overvoid; three aspects of the same thing. Been far too long since I read Gaiman's Endless to remember, though I *think* the Writer and Overvoid are supposed to be the same thing? That's the impression I've got anyway.
You got me there.
I suppose we also have to acknowledge Perpetua as well. She's, from, what, Monitor space?