http://imgur.com/a/vTcYT

Look at the figure standing on the far platform in the first picture. Did anyone else notice this guy?

I'm still just on this mission. I'm thinking it's the outsider observing the players actions, but if it's explained later on in the game, please don't spoil it for me.

http://www.gamesas.com/topic/1416441-hidden-smoke-man/

Assassins, they hunt you randomly throughout many missions. I've played through twice and it seems that, actually, the LESS attention you call to yourself, the less chaotic you are, the more they look for you. I encountered them barely at all on my first playthrough. On my second I encountered them almost every mission. They also get more... observant? in later missions. I was able to easily stealth past a group on a very early mission, despite being near to them. I was actually able to get them near to a certain area where I knew there to be thugs using bottles (noise). The two factions actually duked it out while I watched from a roof top. After the last assassin died (What? Yeah, the thugs won, but there was only one of three left), another appeared on a nearby roof top and spotted me (appeared randomly really close by), ruining my ghost. Anyway, the less havoc you cause, the more they seem to appear.