What does Mr. Van Daan tell the frank family about what people think happened to them?

Mr. Van Daan relays that people believe the Franks have escaped into Switzerland.

About six months ago a high-ranking officer came to the office. It seems he and Mr. Frank grew up together. He promised to help Mr. Frank if it was ever necessary. As I recall, he was stationed in Maastricht. I think this officer has kept his word and is somehow planning to help them cross over to Belgium and then to Switzerland. There's no harm in telling this to any friends of the Franks who come asking about them. Of course, you don't need to mention the part about Maastricht.' And after that I left. This is the story most of your friends have been told, because I heard it later from several other people."

What is the cover story van daan helped make for the franks