The dagger looks impossible to unsheathe normally.
I have, however, thought of a way that it is possible. - Instead of pulling it out, one side of the sheathe is completely open, so you can "swipe" the dagger out of the sheathe. The other side of the sheathe is closed and there is something on the open end that keeps the dagger in place just enough so that it doesn't fall out by itself.