i'm not quite sure if rare treasures are dropping, i did pick up one of my remaining 3 treasures, but i'm still getting mostly really common treasures, hopefully i can find this last treasure for elimination, i'm sick of the camping fest and the power weapon who***, i've even become one in this mode so people won't use it on me!
EDIT: nvm, it's working i just got the last elimination treasure (so 2 rare treasures in 4 games, not bad), just 1 treasure left until i have all of them
Double XP works on all modes, competitive and coop. ND's website says that the increased treasure drop is only on coop modes though. Maybe you just got lucky if a rare treasure popped up in another mode like plunder or TDM.
I finally got last treasure /unlocked Top Hat last night in Co-Op Arena (Oasis).
Uncharted 3 Co-op Certification Center: Uncharted.VarHyid.com