I'm not talking about the contemporary art and not so much about the XX century, although that's where it begins — problems with provenance are very common (2 World Wars don't exactly help) for the works from the first decades of the XXth. But my point was about earlier centuries — XIXth and before, because it was a norm, for example to take the best art from the conquered nation, even from the museums; and that's a well documented and known collections, imagine what happened with personal ones, which ceased to exists with their owner's untimely demise — only for the works to appear a few decades later in a different country. That's something that I know from experience of working in that field.
So it's not only a local problem with, say, British Empire, stealing historical from their colonies. It's something that went on for centuries all over the world.
Take a look at the recent scandal at Ghent museum. It's a little different problem, I wasn't even going to mention forgeries, but still, there is a collection full of them (and, obviously, those works don't have credible paper trail — because they are, well, fake) and a huge museum in a law-abiding country does a whole exhibition on them. This example, by chance, got public, usually they don't.