AI: aware of fleet strength increase?

[catweasel]

In my recent game [1.1.0.9] my nasty neighbour declared war on my empire, to which I reacted by giving a build order of 12 frigates. As soon as this build order was completed, I got an offer to end this war - without any single action having happened at this point.

My fleet assembled rather far from the neighbouring empire's systems, and my diplomacy info shows a "known military strength" of 0 (and I assume they should have similarly limited knowledge of my strength). Three spys are assigned to counter espionage. Which leads to the question: does the AI have more info at hand than a human player would?

[zgrssd]

Maybe they got Long Range Sensors?
Maybe you missed the Exploration ship doing a "Spy on enemy System" mission?
Maybe they still go a transport around?
Maybe they send a spy to get a Operations map?

[frankycl]

In my recent game [1.1.0.9] my nasty neighbour declared war on my empire, to which I reacted by giving a build order of 12 frigates. As soon as this build order was completed, I got an offer to end this war - without any single action having happened at this point.
(...)
Which leads to the question: does the AI have more info at hand than a human player would?

No, they don't have more info and also don't need this (you can check on this easily by switching empires with the game-editor).
I think that it was just a coincidence that they ceased war at the same time you had built your fleet. This can happen all the time (and also happened quite often in my games), independent of what you build or what strength/power you have.
If an AI-faction decides to declare war or to end it is mainly (if not only) dependent on the relations (-values) you have with them at this time - and if your relations changed (in this case increased) before any of the factions had done anything "war-like", it's natural that they decide to end the war immediately (although I don't think that this was the case in your game - see also below).

However, there are also other things that might lead to such odd actions of the AI - like e.g. a special event where they claim that your faction has insulted them, because you hadn't enough info/usage of their language and therefore said something bad that was meant in a completly other way - and this leads also to an immediately declared war (and I guess that this might be more the case in what you had experienced (?)). But the problem is that this event-declaration of a war is independent of the relation-values you had with this faction at this time (/the AI does not check for those if you got this random event), and so leads to an equally odd/surprising end of this war, when your relations wheren't that negative at that time. - This happened quite several times in my games, too.

I also think that this might be the case in your game, because this is the only war that will end without negotiating about it ! - Normally, when you are at war with another faction (independent of who started the war) you / the AI can't just simply end this war, without negotiating about this in the diplomacy/dialog window, and without the fact that both of the factions agree to end the war - this is/needs always to be mutual, and can never be executed by one faction alone.

[catweasel]

Thanks for the detailed answer! (after hundred of hours DW:U it was only my 2nd attempt at DW2)