Rumor bosses appear in the background and are randomly generated. Level restrictions apply to the type of Rumor Boss that will appear (see link below). IF a Rumor Boss exists on the world, you have a chance to learn about it when Gossiping. To the best of my knowledge, the chance to learn a rumor is indifferent to the City / Tavern.
Gossip about Rumor Bosses and Town Hall Quest follow a similar process. Each successful gossip learns a bit about each Town Hall Quest / Rumor Boss in the following order:
1) The Region (general area) of the world. E.g. Northern Varsilia.
2) The Grid (a single map) of the world. E.g. Northwest Sagar Forest.
3) The Location on the map, -or-or in a dungeon. E.g., to the northwest by the mountains, -or- in side Gorx Lair.
4) If in a dungeon, the fourth rumor will tell you the Location inside the dungeon, e.g. to the north in the throne room.
Keep in mind that Town Hall Quests and Rumor Bosses can appear behind secret doors and on easily missed spots of the map.
Wiki link of Rumor Bosses:
https://www.exiledkingdoms.com/wiki/ind ... Encounters