Short form: the AH rules state during the final battle, investigators "may trade items among themselves as if they were in the same location". This does not, however, state that they are in the same location. So, what location are they in?
Reason for question: In a recent game using the DH expansion, Carolyn was working on the "Sealing the Beast's Power" mission as Nyarlothotep was due to wake any moment. Turns out she wasn't fast enough: during the mythos phase of the same turn that she reached the last location listed on the mission (Ye Olde Magick Shoppe) Nyarlothotep woke up. But then I started to wonder: during the upkeep of the first phase of the final battle, could Carolyn sacrifice one sanity and reduce Nyarlothotep's toughness? The DH rules state "the investigator may only discard or spend the listed sacrifice if she is still in the listed area during the upkeep phase". Carolyn was at Ye Olde Magick Shoppe before Nyarlothotep woke up, but what about when the battle starts?
Analysis: a closer reading of the effect on the card ("The Ancient One's combat rating is reduced by 2 if it awakens ") led me to decide that since Nyarlothotep had already awoken that it didn't matter where Carolyn was, it was too late. But this got me thinking about other missions and whether they would be useful during the final battle. And sure enough, I quickly located "The Light Of Reason" which grants 10 clues as it's effect. So a similar situation could arise where an investigator is at the last location with necessary sacrifice when ancient one awakes...
Has anyone ever told you have a lively imagation! This is a good thing, of course.