Someone said that they were alerted to a 'rare spawn' in an area and it was an Eevee.
I can't find anything concrete though. :(
That happened to me this morning in the first quest. A farmer came up to me and told me an Eevee appeared in one of my cities. I only got one Eevee out of the event, so it's not the same as swarms and it didn't have an exclamation point reminding me of its appearance.
I can only assume it's a random occurrence and a rare one at that. Then again, I haven't seen any swarms in the main story yet.
Fair does not exist. Fair is a word from a world of happy fantasies.