Eagle's eye

Does anybody have a clue how to get this achievement? Is it possibly glitched?

I've completed the game three times now, each time being more thorougly than the last. For the second and third time I used this guide as my basis (but still investigated stuff that was not on the list).

Right now, I'm thinking that either the achievement is glitched or I'm missing something so ridiculously obvious that nobody bothered listing it in any guide...

EDIT: I just read in the "Large donkey" thread that re-using the completed save file might mess up achievements. To be fair, I did get Large Donkey while re-selecting the completed save and it unlocked no problem. But could this be the reason for "Eagle eye" not unlocking?

Did you get the Observation puzzle for packing the suitcase? If you're skipping that one, you won't unlock the achievement.

I did this on PS4 (I wrote that guide), as well as on PC, but I can't imagine the XBOne conditions would be different.

As much as I hate to say it...try it all over again on a clean save.