Edited with last location from Grimm Tim & location 2 isnt a hidden.
To get 80% you need at least 4 of the locations.
Use Abandoned Office fast travel.
Head into the offices and take a left into the square area. On top of a filing cabinet you will see a lamp and a painting on the wall. Blow up the wall and head through to another office where the doors are blocked by cabinets. Head to the back of the office and you will see a crate and once near it you should be in the "hidden location".
Use Turn Table fast travel. Not a location but a cool easter egg
GO towards the Utility Corridor where you will require a lamp to see through the darkness. As you progress you will see a window (before turning right and continuing into the AC system). If you look below you will see a lamp below, break through the windows and fall down. This will give you a good feeling of the last dlc with the "swift platform". Turn around and you will see a button to open the door to escape.
Use Lower Access fast travel.
Go towards the Shifted offices and right after entering the offices (from the turn table area through the double doors that the darkness was blocking) you will come up to a messed up staircase that has been partially altered. Look up and you will see a crate and once you jump into the area you will find the "hidden location".
Use AWE Transit Bay fast travel.
Go to the shifted passage about 1/2 way through right before the death ball that you cant kill, there is a green carpet with a painting on the wall. Blow out the wall and you will need to head up into a sideways office. Once inside you will need to move across the sideways room and fall down into a room with a radio. Blow out the wall to the left of the radio and you will see in front of you a clock. Once inside the room you will find the "hidden location". If you wish to do the Easter Egg the clock code is 1/7/7/1/6/4/9/7.
Use Fra Mauro AWE Enterance.
From fast travel head towards the observation deck and look right towards the elevators. There is a elevator on the left that has stuff blocking it, move this and drop down to find a chest and the "hidden location".
Use Eagle Limited AWE Entrance fast travel
From that fast travel, walk straight forward into the circular room with the spinning machinery in it (enemies may spawn in this room). Drop down and turn 180 degrees to see a painting on the wall (the wall is directly under the control point). Destroy the wall and head inside to an area under the elevator shaft. "Hidden location found" plus two chests inside. You may need to bring a light, it's dark as hell (there is one on the right of the wall once you break through). --thanks Grimm Tim