That's interesting. No tears in the plastic? Is the plastic where something could reach up underneath it? From what I understand Raccoons are very innovative.
I just don't know - it's possible the chick originally could have been closer to the wire and got snatched - I don't want to be...
I'm sorry for your loss.
Can you post a photo of the pen where it was housed?
Any chance it may have been near the wire - I have read that raccoons will grasp and pull what they can (I know - not a good thought), but maybe it was able to just get a leg hold and that was it.
Just my thoughts.