It was the one important piece of evidence the Defense couldn’t produce, and nobody knew what had happened to them.
I first saw the movie when I was sixth or seventh grade, and I was not expecting the discovery at all.
A lame movie with a lame spoiler:
Kingdom of the Spiders
Nobody saw it coming because nobody was awake long enough.
My pet theory is that it was Spade who replaced the real falcon for a fake. Knowing how quick the bad guys found about the bird being a fake, I think the others that had the bird earlier would have figured out too.