A passer-by was a person who happened to be walking past someone or something, especially when something unusual would happen.
In 2364, William T. Riker told Natasha Yar that as far as the mythology of Aldea went, he said, "Somehow, as the legend goes, the Aldeans were able to cloak their planet in darkness and go unseen by marauders, and other hostile passers-by who might rob and plunder." (TNG: "When The Bough Breaks")