So I've tested all 3 bandums and found that a missing nin can cap both Sand's Bandum and Sound's. For some reason we can't cap Leaf's. Either its a bug where we can cap sand and sound or its a bug we cant cap leaf. Whatever it is this is one of the bugs we need to one day look into.
EDIT: I tested the 3 village's banners too. Can't cap any of them.