I know this is a bit late to the conversation, but to help anyone that might google this as I did...
When you see the red circle for the location in the Ugamaro mines (Spelling?)
Its in the Northeast of the Red Circle...
The tricky part is you enter the circle from the south, but have to exit the circle on the most west side...
head north, and than theres a small hidden hallway that takes you back into the red circle location...
and sure enough theres a small area that says "Destination", even though the quest says it will be in a coffer...
Its actually a small area you click...
Also to be noted theres a FATE named "Full Metal Alchemist" that spawns DIRECTLY on top of where the location of the Thyrus is...
Its easier to just wait for the fate to end, and run in and grab it.
Hopes this helps any White Mages looking for it...