There are 28 Larry’s Notes locations in Far Cry 5: Lost on Mars DLC. They are text collectibles found at fixed positions. Finding all notes unlocks the ‘DLC: Mars: Martian Journal’ trophy / achievement. You have to pick up all of them before finishing the story or else it will wipe your progress (no free roam after story). There’s a very clear warning at the point of no return, it’s after activating all 19 Antennas and finding all 12 of Hurk’s body parts. Best to leave Hurk’s body parts alone so you can’t trigger anything by accident.
Larry’s Notes aren’t marked on the map. They are all found at man-made structures though (so not in the middle of the desert sand), usually on metallic boxes. When you get close to them they will show an orange interaction icon so they are easy to spot (must have world icons enabled in settings).
The best time to collect them is after having activated all antennas. Then you can fast travel around the map to make this go faster. It’s best to not pick up anything until then. The collectibles remain in the game world after picking them up so it might confuse you what you have and haven’t found. If you run into one by accident, remember the location to not get confused later.
#1 – On a box on top of the Robot Repair Center.
#2 – Up high on a very tall metallic tower (near Bovine Research Center).
#3 – In the dirt there is one single metallic box with a note on it.
#4 – On a box on top of a big metallic platform (east of Solar System Observatory, above ground).
#5 – Underneath a tall metallic tower, on a box.
#6 – On a box on the ark.
#7 – On a box on top of a metallic tower.
#8 – On a box next to something that looks like a small metallic shrine.
#9 – At the bottom of a wide metallic platform. One of Hurk’s body parts is also here.
#10 – Inside the Robot Creation Center on a table.
#11 – On a box on top a tall metallic tower.
#12 – On a single box in the desert (there’s a round metallic platform next to it).
#13 – On top of a flying platform. Easiest way to reach it is to wingsuit from the Pits of Peril Antenna.
#14 – On a box in the sand, just south of the last location (*Huge thanks to SinistralSpark8 for this location).
#15 – Again on a single metallic box in the desert (with another round metallic platform near it).
#16 – Inside the Desolate Dunes Mining Station. Behind the Mining Station you can drop down into a hole, then you land in front of Larry’s note.
#17 – On a box in the middle of the desert. There’s some other metallic rubble around it.
#18 – On a box at the Robot Storage Facility.
#19 – On a box at the southern point of the desert, where you fight two Queens.
#20 – Inside the Hemoleum Research Center, at the lower level where all the green canisters are. You must have activated this center first, then the door to the green canisters opens up. On one of them hangs a Larry Note.
#21 – On a single box on top of a mountain, next to something that looks like a metallic shrine.
#22 – At the north-west entrance to the Arachnid Research Center, on a box.
#23 – In the south-west section of the Arachnid Research Center, on a box.
#24 – In the south-east section of the Arachnid Research Center (inside the center), on a box.
#25 – On the first ‘floor’ of the Terraformer (which is badly destroyed, must jump up once from the desert ground).
#26 – On a single box in the southern section of the map.
#27 – On top of a tall metallic tower. Because your Gravity Belt doesn’t work in this area you must jump to it from a nearby mountain.
#28 – At the Geological Research Center, underground, just where you enter/exit to the console you have to power on with an energy core. It’s sitting on a box.
That’s all 28 of them and will unlock the trophy “DLC: Mars: Martian Journal”. Thanks again to SinistralSpark8 for location #14, much appreciated!