Originally Posted by Gamer5170He's absolutely right.
Unfortunately, even junk is subject to the blessings or curses of RNGesus.
We don't ask about my collection of 37 utensils stolen from bags lying around the city...
... What?
Why are you looking at me like that?
But in all seriousness, yeah, completely random. All I can really tell you is search every bag you see lying around. Probably your best bet. To make things easier, donate to a Control Point to get the detection buff.
I have enough diamonds to start my own Division Agent Jewelry store.