It is actually a side-quest, but either way the piece you probably are missing is on the second floor of the ruined house that dead ends into something like a roof top garden. I know I wasted a lot of time trying to figure out where it was until I realized that there was one location that I had not checked yet, despite my thinking that I had seen everything. The other 4 are pretty easy to find it is the only one that is a little more challenging.
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