OBRkenobi
Chieftain
how long does it take a civilization that is secretly plotting against you to declare war on you?
I was reading that apparently a hidden AI opinion towards you (ie Hostile, Neutral, Guarded, Friendly) is Deceptive, which means that it appears that they're friendly, but they're actually Hostile. There are a few ways to identify this, if you know that they have good reason to be angry at you based on the game's mechanics but no red status show up in their diplomacy towards and they're all green (when they're Deceptive they hold back the reds) or when you're friendly but you can't trade without getting raped.
I bring this up because possibly the XYZ is plotting against you might reveal that they're being Deceptive towards you. Which I guess doesn't necessarily mean that they're going to attack you if you have an adequate military.
Personally, I would take any tip offs as a warning to build up your military. Any time somebody isn't happy with you, you're at risk of being invaded if you have an inadequate army. Only slouch off if you know that everybody's happy and you need your production and gold to go elsewhere.
Yeah, the real test of deceptiveness is what trade terms they will accept.