It goes without saying, that you need to look absolutely everywhere at every location within the mission, but do not forget the fixed locations for the mission (the lab, brass's office, the morgue, interrogation room and the garage).
It is worth pointing out that when you are in the lab, you need to look at/use everything in there. This includes the end of the hall way (left and right) and ALL of the tools used for analysis.
It is important to make sure that you look at the evidence board (far right of the materials lab) as well as the work top (just in front of the evidence board).
These areas are easy to miss and will add to your overall percentage.
Personally, I found that mission 5 was the best to get this achievement, as you have to visit every location that is available, increasing your chances of getting 100%. I must add though, that before going to talk to Beatriz at the end of the mission, my thoroughness was at 97%, it wasn't until I finished the conversation (and the mission) that this was bumped up to the full 100%
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