Where are you going to find a specialized consultant expert enough to judge if its fake who also doesn't know that the painting he is looking at was recently stolen from a museum?
I'm not saying that I know for a fact that this is how it's happening, but it doesn't seem far-fetched. What I do know is that a lot of what's happening in the art world is shrouded by a veil of secrecy.
It's an interesting question, and there are rewards for recovery so it's probably under the same trust calculus of any other criminal venture: pay enough that they don't.