The treaty itself was no secret. It's importance was recognised, all over the world and the sphere's of influence decided. It was headline news those days. Papers published the text of the treaty. It did have a secret protocol that dealt with the actual partitioning, but as I wrote, the very fact that the pact was announced was very significant in itself, as it meant Hitler and Stalin had struck a deal. So yes, dissent is a reasonable way to model in HOI terms what the German population (and army) would have felt if there was no deal and Germany went to war anyway.