AI changing his mind?

[ChaosSlayer]

Something curious happened today.
Somewhere in late medieval era a large fleet of Mongolian ships manifested near my shores.
I counted a dozen of triremes and dozen of galleys, most likely carrying 20+ land troops. They quickly sailed along my shores, going up north to my capital, protected by a lone Warrior. More than that all nearby cities are also protected only with obsolete units. In case of sea based invasion this entire territory would quickly fall.
Now the Khan was mostly Pleased with me before this, and considering he is building his empire on another continent, we never had a conflict of interests. I know that I am more advanced than him and my army pretty numerous and could easily fight off 20 units invasion, if it wasn't already fighting a war against Maya in the far South.
So breaking down in panic, I had to select a large chunk of my units fighting in the South and send them up North. Even with roads it would have taken them at least 6-8 turns to get there...

In the main time the Khan fleet made it all way up to the capital and started circling around nearby island, also settled by me. Then they parked at nearby shore and sat still for another 5-6 turns as my units made their way north. Suddenly the fleet started moving again, this time going down South along the shore, to the edge of my lands and vanished in the fog of war...

All this time the Khan remained Pleased with me and never issued any demands or requests.

 

[Fish Man]

This is really frustrating, too...especially if you can never tell if they're not even plotting and you're wondering if they're going to dagger you. Mehmed did the same thing with half a dozen galleys the last game for quite a while, and I was basically shuffling my ten or so leftover war elephants from city to city, paranoid about him attacking. I was wrong, and there was no danger at all, but for the rest of the game as he gradually teched on and obsoleted my elephants I never got less anxious. At one point the war horns almost gave me a heart attack, but then I realized it was just him picking a bone with his religious archnemesis Joao.

 

[elmurcis]

I have seen many times AI unloading all units inside some of my coastal cities (attack target) after I got 10 turns peace period (at the end of what (s)he will declare on me). So its just about finding where they will go (tile outside borders) and prepare some counter attack force (barrage siege). Sometimes my "calculations" are wrong and they jump to tile far away... moment when reloading seems to be fair

 

[ztrapon]

There are cases where the AI is not "set" on warring yet, so when you pop a new unit, or put defense in a city, it's enough to redirect them. You'll for example see units piling up in one of their cities next to your border, but as you muster up enough defenses, they disappear into the fog and go somewhere else.

But in your case, wasn't the AI just using your territory for troop movement and it happened to look intimidating to you?