... to actually, y'know, try to answer the OP's question. Leaving definitely wouldn't have caused the boss to despawn or somethin'; the only thing that can disappear when you leave a level is items on the ground and outside of vaults. NPCs of any type are untouched, so far as I'm aware under any circumstances.
I'unno if the quake actually deleted the boss, though. Iirc that was
a thing at one point (quake has had a lot of weirdness throughout T4's development), but I'm pretty sure that was fixed (along with disappearing stairs and a few other things). What I can't recall is if quake's (still?) able to stick enemies inside walls. If that
had happened, it's entirely possible norgos suffocated to death (you would have still got most of the XP, I think) while stuck inside terrain. And when that happens, occasionally the items they drop just kinda'... don't show up, instead of getting displaced outside the wall. Forget exactly what's happening to them (want to say they end up outside the map's boundary, or somethin' like that) and it's supposed to have at least been greatly reduced in likelihood, but it's a thing that can happen every once in a while. So basically norgos may have died an ignoble death stuck inside an iced over tree, and left no indication it happened besides a line in the message log about a powerful enemy falling, which is really easy to miss.
Dunno how much it actually helps nowadays (it was made for older T4 versions), but you might consider running this
, which is supposed to help keep oddities like that from happening. Pretty sure most of what it covers/covered has been more or less fixed, but I still get a popup from it every once in a long while in the latest versions.