There are two types of collectibles that can be found within the Mayday DLC.
Classified Intel: These are randomly picked up from searchable items, and their locations change during every playthrough. You will need to pick up a total of 4 of these intel types to unlock them. Only 1 player can pick it up, and it will automatically unlock the intels in order. Any area can contain intel items, and this can easily be boosted solo using the first area and then dying during the first drill section.
Encrypted: These are also permanent laptops that can be found on the map, and their locations are as follows:
- Sleeper Agent: Drill the upstairs door in the first area. Shoot out the window, crawl onto the pipe, and follow it to the left to find the laptop.
- Encrypted: On balcony close to Tesla/HypnoTrap schematics table after clearing the tentacle.
- Witchcraft: In the middle of Area 2 on top of some crates. Use the crates to jump to the top.
- Mutiny: Entering the indoor area on the top deck, this can be found at the back of the room behind the large cylindrical column.
- The Guardian: Final area, on the opposite side of where you enter on the walkway.
Lastly, if you complete Mayday you will unlock the Epilogue Intel,but it is not necessary for this achievement. You can check your progress in the Extinction main menu, and there is a tracker available in the top right corner of the screen showing how many items you have collected.