When at the beginning of the mission you get to the helicopters airfield, run between the helicopter wreck on the right and the wall on the left. An opponent will run out of the room on the left (first picture above) - once you've finished him off, quickly hide in the place from where he ran out - stand on the left side of the door and look around the room. At one of the tables you will find intel (1/3) (second picture above).
After getting to the lab, take optical camouflage and clear the opposite hall. When the hall is empty, get to the stairs and then make a 180-degree turn. On the right side you will notice an entrance to a small room (first picture above). Go there and look for intel (2/3) on one of the desks on the left (second picture above).
Once you obtain the Celerium, clear the room of enemies, then go through the door to the room (first picture above) behind the two turrets, which earlier repelled down the ceiling. By the wall opposite the entrance, through which you got here, a little to the right there is a table. You will find the last intel (3/3) (picture above) on it.