I acquire the game recently, it's so great, I am addictive!
I have a question about the leave button icon. I have done a few mission sand I noticed in the last one that, despite I have done all the objectives and clean (I think) all the enemy forces, my "leave" button is not green! I can leave the combat anyway but I wonder if it's a clue that tells you "he Dude you missed something" or is it because there is still enemy arround or...
Someone have an idea?

I believe that this is because there are some missions that require you to carry a body out of the combat. That action is not checked until you actually leave. Kind of annoying, as I like to clean out the entire sector before moving on, and on those missions, you can never really be sure that they are all dead.

I'm not sure if it's that, in one of the mission I didn't have any objective like this but one of my men said "we have to find the leader of this attack" (secret objectif? nothing appear in the objectives tab). Actually there was no officier and when I finish to kill all the enemy, the button was not green !
But for some other missions it might by that!

As it's explained in the manual,

Red = You can't leave either because an enemy's staring at you or certain mission objectives are incomplete.
Orange = There's still enemies around, but you can leave if you want to. Supposedly there a chance that you may be caught by the enemies before you actually leave, but that's never happened to me.
Green = There's no more enemies to kill. Go away.

I've come across several missions where these rules don't quite apply. There were a few missions where, for some reason, the button won't turn green until I killed all the civilians. And no, they weren't "Armed Civilian" (even though one of them did have a silenced SMG), and at no point did they turn hostile -- even as I sliced them to death. There were also a few missions, namely a few the city random encounters in Germany, where there are enemies stuck in unreachable places, preventing you from killing them and them killing you. These encounters, therefore, can never be completed so the button would never turn green.