I have a few fleets parked around worlds on my border. They are not involved in any battles at all. Yet every now and then, more than a few turns but less than 10 turns usually, one of them will ding a level.



This is consistent and just seems to happen despite my peaceful admiral heroes doing nothing other than park in orbit.



Do heroes get xp based on battles happening anywhere in their detection vicinity maybe? I would have thought it should only be based on actual combat but I can't explain how my admiral heroes are leveling up in my pure peace diplo game when they've never fought.

Your heroes get a base XP of 1 per turn merely for being assigned to a fleet or a system.