In war situation against the AI's I find AI units, mostly Legions, suddenly appearing deep inside my territory (12-15 squares). They seem to appear and then disappear. They will appear suddenly to destroy settler units and other units outside of cities and then disappear. They will also dance around, moving 5-6 squares at a time. Once this starts happening I'm defenseless and get systematically slaughtered. I can't catch them and I have no idea where they will show up.
Nevermind, I figured out what is happening. These AI units are literally doing an end-run and then coming at me from behind before line-of-sight allows me to see them. They then drop back out of sight again and hit me by surprise again. Still some more to learn.