An interesting topic, I must say.
Since the "meeting" of Elwing with Eärendil is described as being an infrequent, unpredictable event, I'd put it forth that a comet might be responsible for the Elwing-sighting.
From the description given, "like a white bird, shining, rose-stained in the sunset", I think something like
this is a plausible representation.