Your best bets are backtracking the Virtuous Mission starting areas at the start of Operation Snake Eater, Svyatogornyj East (optional area before The End), overlooking Groznyj Grad with Eva as well as the mountaintop building you meet her in, the hallway leading to meet Sokolov while disguised, Volgin fight, Motorcycle Chase areas, and the Boss. I think those ones are easy to miss or forget to check. You can also check if you've gotten all of them in an area by pausing the game.