Gadget Finder Achievement
In the prologue, after you enter the main area (the big area), follow the map up and around to the left. You will be blocked by a blue barrier and have to travel up. Keep following it up and eventually to the right.
Eventually you will come up to some walls almost like a castle, continue following the edge of the playable area, it will start to force you to travel down and to the right now. You will pass a small peak and lighthouse above you on the map. The secret item is in the red box next to the lighthouse.