i recommend you to check the confirmed bugs subforum to see the explanation for this. It looks like the AI considers their units as their cultural borders as well, so , if they have a settler ready and a unit scouting farfaraway, they will consider the tiles near that unit as close of home as well ...
Ah, that might explain something in my most recent game I was puzzled about - I'm the only civ left on my continent, Askia and Elizabeth are on the other one. Yet recently Askia warned me about massing troops near his borders! I had all my troops on my continent, my navy on alert round my coastline, several hexes away from his. However he had a frigate out on patrol, and a scout embarked, both passing my coast.
Christ, there's some idiotic bugs got through testing!