Sorry to hear about that, Thumbnail. I hate to loose fish, too, and it seems like sometimes they just up and disappear. I agree, it must have been a bird.
I had to redo my baby turtle pen after the hurricane busted down my fence. I made the pen escape-proof, I know. But I have not yet been able to get a screen cover for it, so I only had three fairly large baby turtles in it, not the new hatchlings. Yet, I am missing one of them. The only thing I can think is that a cat or 'coon got in there and found it and made off with it.
Makes me so mad.
I hope your goldie is just hiding, and comes out in the open soon.