The Witness has many mysterious Collectibles that include strange Discarded Panels with yellow triangle markers scattered about the island.
These panels seemingly serve no purpose, are often hidden or out of view, and do not appear to be connected to anything. However, they can be "solved," even if they don't open any doors. The solutions aren't random and actually teach you an important concept for the end of the game.
They all feature triangle icons - where the number of triangles in a square is related to how many times a line can touch the square.
Below you will find a list of panel locations - sorted by area.
Discarded Panel Locations
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Entry Area - Side Gate
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This panel is behind a locked gate near the Entry Area, which you will need to unlock after unlocking the main gate. Once done, return to the puzzle panel in one of the towers that had two solutions, and enable the other switch that leads out the back of the tower. Head around the fort to find a now unlocked wooden gate, and follow the trail looping around the walls to another door with a puzzle. On the other side of the door you'll find a broken wall and a tree, and to the right is where the panel is lying.
Glass Factory - Roof
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From the entrance to the Glass Factory, head up the path back towards the Entry Area and make a right onto the rocks that lead up to the roof of the place. Head down the stairs towards the water's edge and look below the stairs for the panel.
Desert - Coast
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There is a discarded panel at the very end of the Desert beach you can reach beyond the ruins, where a stair path will take you down to the water's edge. Keep following the beach as it loops back around towards Symmetry Island to find the panel.
Quarry - Roof
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There is a discarded panel at the roof of the Quarry. From the desert head to the top of the elevator at the quarry, take the stairs on the right side and then the stairs on the left (just before the stairway down to the inside of the quarry). On top of the roof walk past the ventilation pipe where you find the discarded panel.
Keep - Blue Life-Size Puzzle
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Go to the Keep and find the area with the statues to begin solving the life-size puzzles. The fourth and final one is the blue life-size puzzle. Near the wall that is closest to the main tower of the keep is a column that holds up some wooden planks. Walk around it to locate a discarded panel.
Jungle - Log Bridge
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There are two ways to reach this panel. The first way is to take the path in the Jungle from the Monastery up to the obelisk. Halfway up the stairs to the obelisk take a detour to the right. You should see a small path obscured by the plants. Follow it to reach a red panel. Directly across from that panel is a ramp made of black rocks. Go up these rocks to get to the log bridge where the discarded panel is. Alternatively, you can find a grey rock trail with yellow mold on both sides from the Jungle Beach to go up. From there it is a straight path to the discarded panel.
Tree House - Burned-Out House
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To get this discarded panel, begin building the yellow path of puzzle panels that leads towards the laser for this area. When you get to a puzzle with multiple solutions (namely the 9th one), follow the image above to end up going straight, leading the panels to a path going left to overlook a burned out house, and a panel at the edge.
Tree House - Green Bridge
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This other discarded panel is located across from another puzzle panel bridge made up of green panels. Just keep going straight - and be sure to answer the multiple answer panel with an ending point leading towards the small house.
Shipwreck[ | ]
Head to the largest hole in the Shipwreck and you'll find this discarded panel inside.
Theater Room - Floor[ | ]
This panel is on the floor in the theater room. This room can be accessed via the Windmill.
The panel is in the left passageway when facing the projection screen.
Town - Shipping Container[ | ]
Inside this building is a shipping container, which is locked with a panel puzzle.
Unlocking the shipping container reveals this discarded panel inside.
Town - Chimney[ | ]
The shipping container isn't the only place in the Town that contains a discarded panel.
Solve the blue walkthrough maze area to drop down the stairs, head up to the roof, and then solve the sliding bridge puzzle to find this discarded panel in the chimney of the neighboring building
Mountain - Waterfall
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Going up the Mountain will lead you to the Greenhouse with flowers of many colors. Keep going u until you're on top of that and head forward up the Mountain until you find a fallen tree you can walk across. Hug the left cliffside and keep going left and you'll discover a long path that will lead you to the top of the waterfall. Collect your Discarded Panel here.
The End - Mountain Elevator[ | ]
Inside the Mountain, you'll pass a discarded panel on the way down from the elevator.
If you miss it the first time, you can take the elevator back up and get it again.