Anyway, let's move on to the episode itself...
Heh heh. Think Mirai said later in the series that it isn't good to be eavesdropping but he himself did so in this episode.
Come to think of it, most fictional characters (not necessarily in toku) usually do this (eavesdropping), and most of the time they come away either hearing ba--er--unflattering things about themselves and/or misinterpreting what they hear due to not being privy to the whole conversation. I suppose this plot device is as good a way as any to set up a conflict or two--as long as the misunderstanding isn't dragged on and on until almost the end.
(Mirai hasn't heard anything bad, because he's not human? Or he only chooses to listen to stuff not about himself? ^^ Or perhaps no one knows him enough to gossip about him?)