What makes Orson's entrance so effective is that he doesn't do an entrance. He just stands there in the shadows, lurking ominously and making Joseph Cotten uneasy. We see his face only because an annoyed Austrian turns on the light. Thus Orson's entrance is that of a rat when we open the manhole cover to the sewer. And then he runs away back into the sewers.
Joseph Cotten discovers the man's presence and takes him for a spy. Why would someone spy on him, he doesn't know, but nothing will surprise him after what he already had seen during his brief visit to Vienna. He calls the spy out, but