There were lots of displaced people at the end of WW2. People moved West to avoid the Russians advancing from the East. Opportunities to move to another country. Opportunities to reinvent yourself.
This is absolutely true. And despite a lot of effort, not everyone could be found. I recently read from newspapers (Sleuthing power unleashed haha.), that they even set points to various police stations, whom collected all the information available of those, whom are missing. Some also posted information about possible returnees and even fallen soldiers They also pointed, that some people could have gone missing even a decade go or more. Another sadder note was, that it was up to the friends and family often, to try and find them. As well as the authorities being annoyed about this, since it meddled with rebuilding and trying to re-establish farming as well as the routes between places. The devastation and destruction was huge, and entire families were lost.
Another note, there was a huge manhunt to out the Nazis (and other parties, including then the Soviets) throughout the world even. The families of the people taking part of the heinous acts could also become targets of assaults, which is why presuming new identities wasn't so uncommon either.
And yes, we know these trials for WW2 crimes still take place. We mustn't forget this.
There is multiple ways how Isdal Woman could play into this, and we've talked a lot here too, about how. I find the courier method intriguing, as there was a letter about this uncovered. Police thought it wasn't really for being a courier, but for something else.
And while the DNA profile is known, it's still problematic. Europe has less tools for mapping it out, than USA does (you cannot use databases like ancestry for example. All DNA profiles from another country needs to be requested separately and individually etc.). While Norway isn't part of the EU, it doesn't help it much either. Also some of the missing people might not have any DNA to compare to, if even files. Some countries might declare you dead at some point, and you might indeed be then completely lost to the history. And if police think you are runaway, they might not open that file in the first place.