When wandering AI units enter your culture?

[Fine_Distinction]

I remember threads like that about barbs, also how AI select their target city, but not on "marauding" unit behavior. Does anybody know the rules?

My testing seem to indicate that they would always go for a worker nearby and enter if a city is not defended (even when they have no chance to see it). They do not seem to be too keen on pillaging improvements, even if it is a border tile, at least early on.