Another update (now tis thread looks like a personal blog >_>):
basikado did some workaround and found a chest for
that key in Maxos Temple: this key for the nearest chest, on the same ladder. Since it's can be opened via lockpick skill (even if it's requires a key) - that's why I didn't found the chest for that key :P
So, chest for key have been found, and, since there is no keys left -
this chest can't be opened too ;<
