Ok here it is since I have to go ...

It was the reflection of the sun in an airplane's cockpit window. The airplane was actually maybe five kilometres away but the extreme brightness played with my sense of distance. The window must have been flat for the reflection to be so bright.

It didn't appear to move because its speed north relative to the bridge matched my speed south -- i.e. the line between me and the airplane crossed (just west of) the bridge at the same point for a long time. The point of reflection must've been tiny. But it was right on me. I suspect no-one else in the ferry saw what I saw. And it must be extraordinarily rare for the circumstances to be maintained for so long. The illusion disappeared after the ferry and airplane made their turns.

The bigger point here is that if you have people self select and report visual illusions it would be amazing if you didn't hear about something inexplicable.

So all this stuff about UFO sightings means about zero.

because it’s the reflection of the sun, and it’s far away, so if you move, the reflection moves too? Which if true lets me guess you live in Manly?

Well done. I was commuting to work at North Head (near Manly).

Reflection off a specific vehicle also moving south on the bridge.

Not bad but I would have seen movement of the vehicle against the bridge metalwork. And the reflection appeared on the other side.