What were the odds that ʻOumuamua passed so close to the Sun? ʻOumuamua passed its closest point at about 0.25 UA from the Sun.

Has anyone researched what the odds were that the first ever spotted interstellar object passed so close to the Sun?

Being the interstellar space almost totally empty, I would intuitively suppose that such odds would be extremely low.
How could it be found otherwise? It is currently (November 2018) only at about the orbital distance of Mars and cannot be detected by the world's largest telescopes.

Such objects are small and only seen in reflected light - they have to get close to the Sun (and Earth) to be seen. There may well be a substantial population of interstellar rocks passing through the solar system unseen at greater distances.