The drop chance for Dungeon Maps is based on the mobs Silver.
This means a mob that not drops Silver will have no chance to drop a map.
As a reference: a group killing veterans in black zone should see about one drop per hour.
Edit: Those maps are tradeable.
Are ALL chests (dungeon and hellgate included) now based on channeling?
If not, does chests have chance to drop maps, since they have a high silver drop rate?