Yeah, check every guard for what weapons they have - sometimes, they spawn with flamethrowers (same for every NPC who spawns in the ranch quest line).
Best defense I’ve found is to either collapse sections in every hallway and clear the rubble (the walls don’t burn) or line the hallways with braziers (if you don’t mind abusing the game mechanic on those).
Well, or just bloody well KILL any NPC with a flamethrower (or debug their stuff away).