[Daynarr]

The thing about AI claiming neighboring system is automatic and hard-coded. Whenever AI colonizes a system it automatically claims all surrounding systems even if he can't see them.

However, I don't believe that AI will go angry over a system that he can't see (in my experience). When game determines AI anger it simply looks at ALL the planets that AI can see and are colonized by you. For example, if I have claimed my neighboring system that has only asteroids in it and the AI has claimed the same system, it won't affect AI's anger. But if AI makes partnership with me (or some other empire that can see my systems), it can go berserk in just few turns.

What AI also hates are ships in his territory (if you don't have any treaty with him). If you have large fleet in any of system that AI claims, he will go angry really fast (the increase of anger is per-ship). This can be the cause of the anger discussed here.