Where do gifts spawn in Eyeaduct?
Here's an image gallery with the gift locations on Eyeaduct.
The gifts that spawn in the underworld give Haunted versions of the same 2011 Halloween items. There's no special difference between Haunted and non-Haunted versions other than the item name and name color (Haunted items are purple, like unusuals).
According to commenters, gifts can always spawn in the underworld or on the main Eyeaduct map. Apparently there is a difference in the "gift has appeared" message between underworld and non-underworld gifts but I have yet to confirm what the difference actually is.