Where is the mask most likely?
In a backpack of some
thief whom you have met anywhere on your way from getting the mask to finding out that the mask is gone [img]graemlins/madhell.gif[/img]
Thiefs in this game can stole everything in your inventory except equipped things. Even quest items, keys, and game critical items. Even the Mavin Sword
Always check what was dropped when you fight thieving monsters!
Searching for the mask now seems to be futile. If you are sure that you have killed everything that popped out from somewhere than you can go and check places where you were before noticing the missing mask. The mask will be on the ground. It will never evaporate. If you aren't sure then the mask can be gone forever.