I am rewatching bsd and I have just noticed. We know that Natsume sensei during Dark Era is in Bar Lupin in cat form, hanging around Dazai, Oda and Ango. And now he is Haruno's pet Mii-chan, so he is close to the ADA.
But I haven't seen anybody mention that he appears in the Fifteen arc too! I didn't notice at all the first time I watched it but now I was like WAIT A SECOND!! He even appears during the second epidode opening. I had to check the markings of the cat and yes, I'm pretty sure it's the same cat.
This is just before Dazai meets Chuuya for the first time.
This is just before the final scene with Randou. The "party" was happening in the second floor that the cat is looking at, and later Chuuya and Randou would fall just where Natsume is standing after Chuuya came flying through the window. I'm sure he watched all the fight.
And now I'm wondering if all this was planned by Natsume, and Mori followed. Because it's strange, he was witnessing everything from the beggining. Mori even mentions Natsume's phrase "only a diamond can polish a diamond". It's too much of a coincidence. Maybe he is the one pulling the strings all the time, he could actually be the one that "created" Soukoku. Or it's just a coincidence and he is just watching...
Anyway, it's even more obvious now that Natsume is always keeping a close eye on Dazai and they wanted us sto know it.