The coffer spawns near the West Atelpa entrance and the main level map where the Wyvern NM and our buddy lizard Amenet spawns.
http://www.speakeasy.org/~sticks/ffxi/treasure/Fr_Kuftal_Tunnel.html
Sometimes hard to blame people for the coffer got opened so often. Kuftal is a popular exp and skill up spot with the common preys all drop the coffer keys, and tons of people go through it everyday.
Best way to maximize your chance to get the coffer is to check ahead how many people are in there... If you see tons of people there, chances will be a lot slimmer.
____________________________
Amanada (Cerberus-Retired) (aka MaiNoKen/Steven)
-- Thank you for the fun times in Vana'diel
Art for the sake of art itself is an idle sentence.
Art for the sake of truth, for the sake of what is
beautiful and good — that is the creed I seek.
- George Sand
A designer knows he has achieved perfection,
not when there is nothing left to add,
but when there is nothing left to take away.
- Antoine de Saint-Exupéry