I don't like "thread necromancy", but Google has limited caps to find things these days...
I have found them in Processing rooms ("PROC" on doors, also marked as such in Terminals), in (locked) lockers.
So Inara is correct. They list buildings which have "PROC" rooms as possible locations, just not mention the room type itself.
I obviously do not have big "statistic", but since I have found all 5 in "PROC" (different systems and station types), that is unlikely pure luck.