You only get reports of battles that occur in a region in which one of your nobles is. If you have a noble in the region, even if that noble does not take part in the battle, even if they have no troops, then the realm gets informed of the battle. No nobles in the region? Then no battle notification. That includes regions within your own realm. Granted, this is not something that happens all that often.
Hm; interested. I would have assumed we would get a report. Well, now I know what happens when you assume...
