Interesting question, and a tough one at that. I have never personally read or heard of any accounts from North Koreans who completely escaped the regime's notice, although it's 100% possible that they're out there and I don't know about them. However, I'll qualify that even further and say that Western historians and analysts would have a hard time getting their hands on stuff like this. It's difficult to get verifiable information from within North Korea about anything, much less people that the state (by definition) doesn't know much about.
So I can't really answer your question with any real degree of certainty. What I can tell you is that these people would probably be very rare if they existed at all, because North Korea was -- and to a significant degree remains -- one of the world's most effective totalitarian states.
So let's say you're a theoretical family of North Koreans living in a remote area of the country during the Cold War. And let's also say you read a few history books before the government banned them, and have an inkling of what totalitarian states usually do to people. You're not an idiot and want to avoid the heavy hand of the Kim regime as much as you possible can.
However, you've got to find a way to circumvent the following:
- The songbun system: Literally everyone in North Korea was classified according to their perceived loyalty to Kim il-Sung. Privileges like good jobs, the right to reside in a major city, and access to the country's best schools were doled out accordingly. This status was hereditary. If you were lucky enough to be the son or daughter of a man who'd fought alongside Kim against the Japanese in Manchuria, your family had it made. You probably had a nice apartment in Pyongyang, your kids could go to prestigious schools like Mangyongdae Revolutionary School and then Kim il-Sung University, you weren't hurting for food, and you had the right to retire early. By contrast, if you were the son or daughter of some poor bastard who'd been a landlord in Korea before 1945, you weren't going to get any of this, and neither were your kids or grandkids. There were a few ways to raise your songbun (and North Korea watchers noted that there was effectively a form of songbun inflation as time passed), but far more ways to tank it.
Now, if you're living on a mountain somewhere and have as little to do with North Korean society as possible, songbun won't play a role in your day-to-day life. However, your background is going to become relevant if you need to send your kids to school, apply to university, apply to become a Party member, get a job, etc. As any hermit could tell you, the reality of life is that you will eventually need to engage with society in some respect, even if minimally. And North Korean society already knew everything it needed to know about you based on whatever your family was as of August 1945.
(As an aside, it was sickeningly common throughout the Cold War to read about North Korean soldiers or sailors who committed suicide rather than face capture by South Korean forces. Why? Because the families of North Koreans who died rather than betray the Great Leader would instantly be elevated to the upper songbun tier and would have huge privileges for life. For most people, the calculation was simple, and the barrel of a gun started looking pretty friendly.)
- Schools: I have no idea whether anyone in North Korea engaged in what we would call homeschooling, but it was certainly not standard practice. In most North Korean families, both the mother and father worked outside the home, and during the day, children were given over to the care of state schools and youth facilities. Primary school was compulsory and students were automatically enrolled in the Children's Union. If the state knows our mountain family exists at all, it'll probably have its claws in the kids at the very least.
- The Public Distribution System: This is the big one. For decades, most of the food and clothes that North Koreans had were given to them by the state via the Public Distribution System, or PDS. The rations allotted by the PDS were rigidly stratified by age, sex, and occupation, and not always very generously. (However, that's a long discussion for a different day, but it plays a role in why so many North Korean women worked outside the home.) If you were completely cut off from the PDS, you were going to have a very thin time. Our family living on a mountain now has to find enough arable land to grow crops for themselves (which is going to be tough, because ... they're living on a mountain), has to have enough threshing and storage space to process, dry, and store grain, some means of preserving and storing other forms of produce, and will probably want to have some livestock as well because cabbage gets old after a while. However, farmers in NK were under a strict quota system and couldn't just sell seeds, animals, or produce off their farms to someone who wanted to get into the business. They were permitted small holdings to grow stuff for their families or the limited markets available, but the yields simply augmented what you'd get from the PDS. Oh, and stuff like tractors, oil, gas, and fertilizer were exclusively distributed by the state as well (including the very first tractor that North Korea ever built and then subsequently discovered could only run in reverse).
- Mandatory military service: Young North Korean men typically entered the army at 17 or 18 and spent a decade or more serving. The only people who consistently escaped this responsibility were people from good families with high songbun who could get out of it, and university students. However, my memory's hazy on this last bit; I don't recall offhand if it was all university students who were exempt, or just those training in fields that the state deemed high-necessity.
- The inminban: Everyone in North Korea was automatically enrolled in local neighborhood groups called inminban. Think of the worst, most intrusive, draconian, and just plain fucking evil homeowners' association in the world, and you'll get pretty close to what an inminban is. So who's running it? An inminbanjang, a lesser demon shat into existence by Satan himself. She -- and it's nearly always a she -- is typically middle-aged or older and isn't otherwise employed. She has all the time in the world to make your life a living hell. She's equal-parts bureaucrat, garbage collector, neighborhood gossip, street sweeper, and government tattletale. The inminbanjang is entitled to know if you've got guests, and their presence has to be noted and reported. Someone's knocking on your door at midnight? It's probably her and the local police just checking to make sure all's shipshape and that nobody's tinkered with the radio trying to get a foreign broadcast. Oh, and if anything's amiss, she'll report you to the authorities, and you and your family are going to the camps. Yeah, so pretty much Gladys from your HOA, with all the power that Gladys secretly dreams of having.
- The desirability of remote real estate: Kim's paranoia over spies and enemies meant that many military facilities, nuclear sites (as of the 1980s), and concentration camps were deliberately located in remote areas. This certainly wouldn't have eliminated all remote areas for our family here, but it would have eliminated a lot of sites with easy road access and water availability.
- Strict control over residency and travel: Official travel permits were needed to leave your home county, and the inminbanjang had to be notified of your trip. The inminbanjang of the place you were traveling also had to be notified of your arrival and where you were staying and when you were leaving so she could report your presence and national ID number to the police. Train and bus tickets were issued only to people with signed and stamped permits, and a closer eye was turned on any travel to major cities, particularly Pyongyang. The capital was the showpiece of the nation, and residents generally had better homes, higher-quality grains, and more access to the best schools and government services there. You can't just visit because you feel like sightseeing, because the government's afraid you're going to try to move there illegally. Pyongyang residents had special stamps on their national ID cards, and woe to he who was caught in the city without one.
What a hassle, right? Our mountain family has decided to avoid public transport and just take their car. Except they can't, because they don't have one. Private cars were (and remain) an extreme rarity in the North Korean population. Owning one would have been the broad equivalent of what owning a private jet in the West is like.
All in all, it's certainly possible that there were people living in the middle of nowhere in North Korea who escaped the system, but it would have been very hard for anyone to do it permanently. The state very effectively managed its population from birth -- even before birth, if you count the hereditary nature of songbun -- and you would have to be almost completely cut off from all North Korean society to avoid engulfment.