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Great to see so many suggestions, this became a bit of a quiz didn't it! :)
It is not for hydration, there is a pouch for that inside the pack, the top of which can be seen in the photo as Marion pointed out.
I think it is probably just for removal of the pack from the frame, if you are suppose to do that, or perhaps just for assembly in the first place. It is odd that it was unzipped when I got it though.
Yes Grumpy, it would useful to email Osprey really ;) thanks for that!