A fairly easy mission, now that you've got the Sneaking Suit.
For an S rank run, equip it for this mission. From the waterfall map, head to the next map ("Bottom of cliff"). Hug the left side of the map and you will not trigger any alerts. In the next map ("Top of cliff") hug the right side. On the final map (AI Lab) hold up the first soldier you see. Then take the left side of the map, holding up the next soldier. You can walk behind the ones in the middle. 4:00 is good for the S rank.
Next: Locate the ID Card