I have never heard of any such thing.
Can you show us something please?

I heard it from French teachers who claimed to have read those newspapers with their own eyes, and from many other sources. Actually the idea of Germans invading France via Belgium and Luxembourg was obvious enough, so that the rest of the world would have been incredibly stupid not to have it. The tactical details are not important for my argument. (Rumours on the net actually claim that these were known to the French military as well.)

If anybody seriously challenges this statement, I can google for support, though I do not know which authority will convince whom.