Why did the Nazis find out that they are hiding in a secret annex?

It was someone (unknown) and we probably will never find out because the person is probably dead and we can't find his DNA. It could have been Meip, Jan, Victor (etc.) for all we know, but I doubt it was them. Or someone will not speak up about evidence. There are many possibilities to what happened. But I really think it was that burglar. But anyway what happened was someone found out and called the "Green Police", Nazis, or Gestapo. (etc.) Hopefully I find out sometime in my life...