The prisoner was in one of the northern cells; the one with the false wall is to the south east. Enter the far eastern cell and look for a couple vertical wooden beams. Check your map; it will show you where you are and where the passages are, though it is not necessarily clear about which wall is the false one. There are only two cells you can open to the south, though, so the options of where the access can be are fairly limited. I think for the first wall you just head straight back in the south eastern cell, IIRC.
