I don't think anyone is calling for a "NPC session." The idea I'd that the world keeps turning even if the PCs decide not to engage. As it should be.They do, though. What the PCs look at is what’s in play. If the PCs aren’t involved, then nothing is happening with those NPCs. When they disappear from view, you can use any method you like to determine what’s happened in the interim, but it’s all abstracted.
Not unless you do the equivalent of cut scenes that don’t include the PCs.