Der Mann mit dem Laubfrosch (1929)

An unknown guest checks into an obscure hotel. He is a burly but well-dressed gentleman carrying an umbrella and a tree frog in a glass. This man spends most of his time wandering the hallways and staircases of the inn, ostensibly to hunt for flies to feed his pet. But quite obviously, there is more to it than that: the man also listens at doors and peeks through their keyholes.