Probably none were really "Hidden". People that were practicing Jews were found out pretty quickly and easily, and at first didn't really have anything to fear from the Nazi regime. That changed pretty quickly and we know what happened from there. "Mischlinge" weren't exactly uncommon. Bryan Mark Rigg, historian and author of "Hitler's Jewish Soldiers: The Untold Story of Nazi Racial Laws and Men of Jewish Descent in the German Military" suggests there were as many as 150,000, citing documents from the era and interviews with many of them.

Have a link to a review of the book with comments by people in relevant fields: http://www.kansaspress.ku.edu/righit.html

Double Edit: Lots of shadowbanned people trying to post in this thread?