Well, first off I'd say that you were lucky to get level 5 Casket detection. I spent about a week in CN's basement and only got it to Level 3, while some of my other buffs were level 11. It seems to level up about as quickly as Attack speed.
Secondly, they *are* that rare. My week in CN probably had 30-50 brown caskets a night, and I never saw Cocoon Band. It took me about 6 hours in R.Pass to get a Grapevine cape, which remains the only brown casket item I've gotten. I'd be surprised if the drop rate was higher than 5% per brown casket, likely between 1 and 3%.
Not very promising, considering I'm about to confine myself to Bost. Ob. (lolthat place) for Melaco Mittens. I'd just buy them, but there aren't ever any on AH.
