Rivers didn't know very much. Apparently, Col. Hall disappeared just after the fall of Berlin. The only thing they knew was that he requisitioned a truck and drove it away himself. That was the last time anyone saw him.
He also said the Army wanted to keep this as quiet as possible. That's why they weren't kicking over too many rocks to see if Col. Hall slithered out from under one. It would be embarrassing after the victory the free world had just won to find out that an American Army colonel was cozied up in bed with a Nazi...