I may be wrong, but I believe the "clue in the mystery" achievements are slightly glitched. To get them, I think you need to play through the game with an uninterrupted save. Because I have the game completed on Legendary, but do not have the Super Sleuth achievement, I am assuming this is so.
ATM - Legacy
Proud obtainer of the MJOLNIR Mark VI Recon Variant (Recon Armor - 9/22/2009, 12:58 AM ET), and the "Mile High Club", "Key to the City", "A Monument To All Your Sins", and "Criterion Elite" Achievements