At first, it may look like that little blurb is an achievement. If you look closer, it seems to indicate some sort action by another character or player. I can't tell if this is online interactions with other players or some other AI-related notification. Thanks to Vinniebrock for the heads up!
UPDATE - We have another look at the messaging system. It appears to be related to in-game content. Still, with the faceless icon that pops up next to the message, I feel like this could be involved in Miiverse activity. Thanks to MazKiller for the heads up!