I think what the OP is saying is that if you get a key and use it at a jail but DON'T LOOT THE PRISONER ID CARD, you still have the quest and can reuse the same key on another jail, essentially allowing you unlimited access to greens and blues. I seriously doubt this is the case but I will have to try it out on a toon who hasn't done the quest yet. I've always looted the card for consortium rep, but if I can use one key to access multiple prisons and get a green or blue every time, hell that's a nice exploit and +rep ya for it.
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this might work because you DONT always get a mob in the jail, sometimes you get an NPC who gives you rep from a certain faction for "freeing him/her" in which case you dont get an ID card. The game mechanics might let you resuse the key when a jail is opened and no card is received, assuming the quest is still in your log.
I farm this area daily for keys and get 4-6 an hour. I know once the quest is done, the key disappears everytime you open a prison. but if you still have the original quest, it might work.