The first two examples in your question ("I could see a dark shape in the street [outside]") sound pretty much the same to me as a British English speaker, and both sound natural. The only real difference is that one clearly implies that the narrator is "inside" without needing additional context:
I could see a dark shape in the street
The narrator's location is unspecified.
I could see a dark shape in the street outside.
The narrator is not outside: they are likely inside a building, vehicle, or similar.
In practice the sentences are more or less identical, and are both perfectly idiomatic.
The third case is a little different:
I could see a dark shape outside.
The shape may not be in the street (which probably implies being on the road or pedestrian pavement), but -- as in a horror movie -- clinging to one's window, building, or in the sky. The narrator is still implied to not be outside.