That doesn't appear to be how the system is supposed to work.
There are several layers to a rumor:
1) Name and type of a monster
2) General area of the Kingdoms
3) Map Name
4) Area of map / which dungeon
5) Area in the dungeon
As you learn more about the Rumor Boss, you should get increasing detail of the exact location of the Rumor Boss. An "unknown area" is supposed to refer to not having heard the area yet not that you haven't been to that part of the map yet. Think of it as someone telling you they lost their phone, but not sure where they lost it.
There's a few hiccups in the system that need to get reported and fixed so if you come across anymore odd issues with "unknown area" report it for review.