This was found in the map High Noon, which is a remake of Hang 'em High of the original Halo.
The creator of Hang 'em High, Peter Marks, apparently died in 2009 due to Cancer.
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F cancer HARD
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I couldn't find it myself, it was hidden in the bitmaps since all bitmaps from Reach maps can be extracted.
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...But where is it located on the map? And that easter egg succeeded in making me sad :(