Silent declaration of war.
Twice in my current game, one civ has attacked me and there was no popup from an advisor saying they declared war. In the first case, it was obvious. A lone tank was moving through heading for the other side to attack the enemy. It turned an attacked one of my cities about half way across. In the other case, I was being attacked by one civ and they were bombarding my coast. The other civ, which I was at peace with, started bombarding, too. It wasn't immediately clear that it was happening, because the civ looked like its ships were just random-walking while the first civ was bombarding. But, they joined in the fun, too.
Any reason why I wouldn't see an advisor popup and say that a civ as declared war?
Civ3 1.29f
Edit: The civ was in a Mil Alliance against me before these 2 incidents, I think. But, there was no MA when the silent wars occured. I have not been in any MPP this game.
I guess vigilance is the watch-word.