What happened was the Nazis stole the art and then it was scattered into underground caves, warehouses and office buildings. When the war started to go poorly the items were distributed to lower people to keep them safer from bombings.
So what happened is art that started in a Nazi cave ended up in a factory warehouse and then pieces went to the managers or employees. So you have collections ending up in a families hands after the war and as the children grew to adults they assumed possession of the art and they feel they can't stand up and turn in the art to the authorities. These people couldn't destroy it, couldn't sell it or give it away so it became a burden in an odd way. They were afraid to turn it in and possibly bring disgrace to their current family.
But eventually the art will make its way back into the proper museums and owners hands. Possibly the saddest thing is that these works have been hidden from public adoration for nearly 70 years!! Even after 69 years, Germany still has not recovered from the war. The destruction was horrific but the Soviet occupation and shame from the rest of the world has left scars.
I'm hoping soon for a Germany that is not so self-conscious and stigmatised.