I'm looking for the title of a story in which aliens have visited Tranquillity Base and have taken the discarded backpacks used by Armstrong and Aldrin for their space museum. The story is told in a series of e-mails (or the equivalent!) between two groups of the aliens, who have learned that Mankind is about to return to the Moon and don't want any indication of their visit. They instruct the second group to create duplicate backpacks and put them in place of the originals. Unfortunately due to a misunderstanding of how the Apollo programme worked, the second group create three backpacks and place them at the foot of the LM ladder!
The story was part of an anthology of stories set on the Moon, published as a book for one of the Apollo 11 anniversaries - most likely the 25th or 30th one.
