You can search for missing items through mission select at any time during the story or after completing it. The intel collectibles look like thin laptops with a blue display.
You can view how many intel you have found in each mission under mission select. Collect all 45 intel laptops to get the 'Ferrum Absconsum' achievement. There are three intel to collect in each mission. Search the indicated locations to find all 45 intel collectibles. Successfully complete Round 11 to view a special cutscene showing the horde of zombies as they overwhelm the area. A DNA Bomb will then drop, infecting the enemies and transforming them into zombies for Round 11. Play the Riot map in Tier 4, and complete Round 10. To reach the secret zombie round in Exo Survival mode, you first must unlock the Riot map in Tier 4. Successfully complete 100 total rounds in the four Tier 3 maps to unlock Tier 4. Successfully complete 75 total rounds in the four Tier 2 maps to unlock Tier 3. Successfully complete 50 total rounds in the four Tier 1 maps to unlock Tier 2.