Here's one I missed in my first playthrough -- an Alien Abduction Warning Sign!

Nice! ^^

Here's one, too: a bit south-east to the camp in the "middle" of the world map (don't know the correct English name for it) there is a house, only a hermit lives there. On the 1. floor there is a group of people that played "The Blue Eye", a reference to "The Dark Eye" (Das schwarze Auge in German).
Actually the group of dead people are members of Rocket Beans TV, a popular german community that do let's plays, various shows and other fun stuff on YouTube. Even some quests are dedicated to them ;)

When you meet Ragnar there is an option to ask " Where can I get an armor like yours ?" its a reference to Gothic 2 , where you can ask paladin Lothar about it . Reaction is more or less the same .))

Looked like a sniper from the inside... ^^

I like the skeleton trying to bite through the bars in your pic.