Neonchameleon
Legend
However, let's say I look at a map of Canada/USA. Vancouver, LA, Chicago, Montreal - they're all there just like they should be. But the PCs are in Lake Geneva Wisconsin, so I zero in on that bit of the map and bring it in way closer. The fact I'm now looking only at the map of Lake Geneva doesn't invalidate the map of Vancouver's existence, nor does it invalidate Vancouver itself - it's still there, even though I can't see it on a map of Lake Geneva.
Same thing with NPCs - the 5th-level NPC Cleric the party bought cures from last night was a 5th-level Cleric yesterday morning and is still a 5th-level Cleric today even though she might - for all we know - never interact with the PCs again. The laws of the game dictate what her being a 5th-level Cleric represents in terms of abilities, h.p., etc; and she's reflected as such on the map whether you happen to be looking at that particular bit of the map at the moment or not.
Put another way, if a bar brawl starts before the PCs walk into town, then the PCs arrive and interact with it somehow, absolutely nothing should change about the parameters of the original brawl. Sure, more dice are going to get rolled if the PCs interact with things than if they don't, but from the point of view of a participant in the original brawl the presence or absence of a PC (who, for sake of discussion, does nothing but stand back and watch) should make no observable difference at all to what happens.
Lan-"brawling with a 5th-level Cleric in Vancouver"-efan
The above is all based on two premises, both of which I find artificial.
1: Levels have a direct rather than an indirect meaning in the game world
2: Everyone develops the same way.
The NPC cleric will always be someone who has been empowered by the Gods. He won't change. But the Gods don't normally do production-line empowerment, and unless you actively go adventuring becoming higher in the favour of your deity does not automatically equate to getting better at wielding a sword. The term 5th level may refer to his level of initiation in a mystery cult - but does not necessarily refer directly to anything in the gameworld.
The NPC cleric is a person with relationships, skills, and divine abilities. And ultimately is too complex to trap in a simple statblock. Any more than writing Lanefan, Level 0 CN Human (or whatever) captures the reality of Lanefan.
And I wouldn't want to straightjacket you to the rules of any RPG.
As for the bar brawl, very little changes whether the PCs are there or not. Or everything changes because the PCs are scary interlopers.