Here's another one from the DRIVE-IN MOVIE CLASSICS 50 MOVIE PACK.
The title should really be: "A bunch of European actors drive around in the fog and eventually meet at a mansion. They talk a lot and nothing much happens. Then the Burgomaster started dozing off and . . . Zzzzzzzzzzzzzzzzz . . . The End."
This one had potential, but they left it behind somewhere else. The premise sounded interesting, but, as you said, they just let it drone on and on without much developing. When they do decide to divulge the mystery, it is a letdown, both for the red herring aspect and that the viewer wasted so much time with this film to have it come down to that.