The 5th-level PCs are burgling someone's shop in the dead of night.
One fails her move silently roll and so here comes a thoroughly typical constable.
Now: is this some 0-level schmuck? Or a genuine obstacle?
The Old School has no single answer to what level average joes should be. Some would say that the PCs and their usual foes are special, and above such run-of-the-mill locals, and that's what 0-levels are for. Others--the authors of City-State of the Invincible Overlord, for example--would say that if just surviving and accumulating cash should give you treasure, then everybody should have levels.
Basically, I'd say it depends on your subgenre--is your game a Weird Tale, or is it Noirish?
I am helping James Raggi edit the Referee's Guide for his upcoming Weird Fantasy game.
Also, I'm in middle-america today. In a hotel room.
Most of Raggi's Referee Guide is simply solid Old School DMing advice, but there are peculiarities, places where I notice his specific sensibilities emerging. Specifically his rules for setting-design as they relate to promoting The Weird.
Most of this town I'm in (town? it starts with a sign on a highway "Welcome to..." and goes until you hit another sign. "Zone" seems more like the word I want.) is flat expanses of grass bisected by straight 4-lane roads. Roads, grass, and trees--typical midwestern trees. Green. Other than that it is--so far as I can tell--entirely malls. The malls are 90% chain stores. If you've ever driven cross-country in a car (or live in a place like this) you'll know I am not exaggerating.
Raggi's conception of The Weird is built on a basic horror or classic Surrealist model--life is banal and quiet, and then The Weird emerges into it, and that's, essentially, the plot. The Weird itself is unquantifiable (Raggi has no standardized monsters, and advises against them) and untamable (Raggi consistently militates against--in both his book and his published adventures--magic-as-technology).
What's Weird in this town in Ohio I'm in is not anything that might be part of the town's description or anything apparent from the air or from driving through it or anything, as they say, on paper. What's Weird is what's hidden and personal. For example, at the mega-sized antique mall: This vintage Art Deco whiskey bottle I found in among the tea cozies that's shaped like a penguin with a screw-off head and a pattern of gold Pollock-drizzle over its see-through stomach that plays "How Dry I Am" as soon as you pull it off the shelf is weird, the pair of King George tax-stamped pre-Revolutionary War bone d6s I almost bought (until I realized how tedious I'd sound constantly saying "these are genuine pre-Revolutionary-War d6's" every time I lost initiative with them) is weird, the number of Nazi daggers and swastika flags lying casually around the place is weird.
In other words, the DM of this town has read Raggi's game.
Who hasn't? I'll tell you: whoever thought up New York City, Los Angeles, Tokyo. etc.
Now, things are not unweird in large cities, but not Weird--at least not the way Raggi means it. There, the default pulp literary form is not the Weird Tale, but the Noir. "Noir" not necessarily as in blondes and .45s but "noir" in the sense that the city itself is a character, not a backdrop. The underlying assumption of noir is: everything is weird. Or at least exotic. Everything is some new kind of strange--telephone booths where people always get shot, bribeable doormen who've seen appalling things in every door, hotels whose signs practically proclaim "nothing savory ever happens here". And: The characters must use this exoticism to survive. Sam Spade must bribe the doormen and consult the pimp and shadow the hotel detective. The noir city is romanticized and exoticized, like a jungle, and its inhabitants must, like a jungle tribe, learn to use it against itself.
Noiry ideas extend to many genres that are not necessarily crime stories set in 20th-century cities through the idea of accumulating a density of exotic details that the characters can manipulate. Fritz Leiber's Lankhmar is noiry, but so is Jack Vance (each place runs on weird rules carried out by weird characters to weird ends), so is William Burroughs, and so are all esoteric urban fantasies where the exoticism of the story is an extension of-, or merely an examination of-, the exoticism of the locale.
Weird Tales draw you from the normal inexorably toward One Central Weirdness (and then, often, kill you--otherwise the effect is diluted, like when you create a Call of Cthulhu character that survives so many adventures it can say "Oh, never mind that, that's just Shub-Niggurath.") Noirish Tales, on the other hand, have different weirdnesses going off in all kinds of directions. (And the noir protagonist is often so used to the weirdness s/he's tired of it. And that tiredness is part of the story.)
Point is: the Weird Tale as Raggi is writing it defines what's going to be exciting as forces opposing or alien to the human landscape the PCs normally inhabit, while the Noirish Tale defines the exciting as an exaggeration of the human landscape the PCs normally inhabit.
The Weird Tale is especially accomodating to: desolation, slow intimations of terror, sudden reversals.
The Noirish Tale is especially accomodating to: satire, expressionism, and post-modern confusions of all kinds.
(There are other options, of course. David Lynch and his imitators--and, I am told, Spanish writers of the esperpento school--frequently work a third position--one where everything in a seemingly familiar and detail-less landscape is a little strange. Twin Peaks would be weird even without Laura Palmer and Bob, Frank from Blue Velvet was just one of many freaks dotting Lumberton's landscape. The idea there was: what looks bland and calm is, not very far under the surface, all weird. In fact, its bland calmness is in itself weird. Unlike the Weird Tale, often this weirdness has nothing to do with the plot.)
Anyway, back to our NPC constable--in a Weird Tale, this constable (and almost everyone else in the city) should be 0-level. And, not only that, but there should have been nothing worth stealing in that shop anyway. (Unless the GM has put The Central Weird in the shop, of course.) At any rate, it should all be over quickly so the PCs can get back to being slowly but inexorably sucked toward the Slumbering Central Weirdness.
If you prefer things to be Noirish, then the constable has levels--as does every bureaucrat, lunatic, and merchant in the city, and the PCs will have opened a can of worms by burgling that shop--no matter what level they're at. Because robbing shops and making getaways and hiding in alleys is the kind of thing they should be doing and spending time trying to figure how to do right, because the landscape itself is the enemy, and they are at war with it forever.