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Just did this myself the other day and ran into similar problems with the map markings. I found the 3rd set of cells to be on the small map with the lower case g, e, f on it. The connections to these unmarked cells are on the 3rd map of Arrapago Reef.

Easiest way I found to that 3rd map was to return to WG and use the Dvucca Isle Staging point. From there, go the entrance marked Arrapago Reef 2.

The cell room with the guards is purely for the mirror, the guards do not mean the prisoners are there. My buddy and I found this out the hard way. =(

You should be able to target the prisoners from the door to the cell room. If not, then they're not there.

Hope this helps, good luck!