The limited edition of Halo: Reach, released in 2010, came with an ID card for scientist Catherine Halsey, who keen fans will know as the architect of the Spartan Programme and was the brain that A.I. Cortana was based on.
The I.D. Card recently gave up a secret (not sure if it is really an easter egg) - place a blacklight over it, and image of a younger Catherine Halsey appears. The blue hue maker her look Cortana-ish – which is presumably the intent given their indelible connection.