I've completed this mini mission twice now and have noticed what appears to be 2 secret treasure areas.
The first I have accessed by entering through a tunnel with a broken grate.
The second however is puzzling me. It is in the first large room with 3 guards on the lower level and 2 up above. Within a caged area there is a treasure box and a glowing lever similar to those in the various hidden seal missions to get Altair's armour. There appears to be an access area within the cage area where Ezio could climb up into.
Has anyone completed this? I looked around extensively and could find nothing.