Clarke, Arthur C. "Passer By" (vt "Passer-By"). Infinity Science Fiction, October 1957. Collected The Other Side of the Sky. NYC: Harcourt, Brace & Company, 1958. See Internet Speculative Fiction Database for translations and reprints.
Listed in Benford and Zebrowski's Skylife: Space Habitats in Story and Science.
In a blog post, "Whole World’s Upside Down," reviewing The Other Side of the Sky, James Nicoll comments briefly: "A curious object seen in passing hints that humanity is not alone. If he is to reveal what he saw, the sole witness would have to explain why he was where he was at the time.… " Cf. and contrast Clarke's Rendezvous with Rama.
RDE, finishing, 14Jun22