An idea (I don't know if this is a little or large tweak so I posted here)
The Proposal
Give the option for the AI to react to potentially hostile or neutral race build-up near their worlds. First by adding a race reduction to relations (much like a weak military makes a race more inclined to dislike you) and secondly by issuing a warning.
Why it would be useful
a) Give the impression of a more life-like AI that actually gets ticked off with people who are where they shouldn’t be.

Gives the AI a chance to act negatively to a race dangerously close to its worlds for any length of time (with the exception of an ally).
An Example
The terrans are controlled by the player who is neutral with their neighbours the AI controlled arceans. The player wants to invade the arceans in a surprise attack and the gullible arceans aren’t prepared because the humans are neutral with them:
Trade ++
Close borders –
Our military ++
But the humans now have a fleet dangerously close to an arcean system they have no worlds in and no resources near, where they are building a few ships up at a set rally point. In real life we all know the arceans would say ‘look buddy what the hell do you think your doing with those ships!!’ but because the AI thinks the terrans aren’t hostile he just sits there and bang five turns latter arcea is destroyed and they are surrendering.
Why not add a large reduction to the relations above if a race is dangerously close to a place it plainly shouldn’t be near:
You are in our space! - - -
After applying this for a time the arceans would contact the terrans and say something like:
‘Your ships are very close to our space friend…move them. You have been warned!’
If the player ignores the arceans the reduction will continue and the next message may well be a declaration of war or at least a massive cooling of relations. If they agree the reduction stops when the ships are moved and relations can be repaired. Either way the computer is far more suspicious of the greedy terrans than he was before!!