This guide explains how collectibles work in the Underground DLC.
In the Underground you can start Operations. The Mission design for each operation is different. The Collectibles are found inside these random generated missions so they don't have a fixed location. For example you can not say Audio Log #02 is found in mission abc behind container xyz.
Every Phase Operation has 1 collectible and this collectible is located in non combat areas and close to the restock crate. These restock crates are mostly in the middle of a Phase Operation, sometimes in the beginning of a operation, so if you find a restock crate look for a collectible nearby. (open the map and look for the collectibles symbol)
1 Phase Operation = 1 Collectible
2 Phase Operation = 2 Collectibles
3 Phase Operation = 3 Collectibles
If you are close to the end of the 80 collectibles and needs only 5 Photos for your achievement as example it can be happen that are no collectibles in the operation!
Currently the tracker is heavenly bugged, just collect so long you get your achievement.
Keep in mind that this is what I found out after the first day of the Underground DLC Release.
PS: I will collect all collectibles with a different GT. I only post here with this GT the solution (Achievement not yet won display)