What I've seen is that the AI will send settlers on a boat or through other civ's domains to find a quiet patch far away where they can create a 'I'm not dead yet' city or three.
Originally posted by Malys Faisent
Right before I claimed the last Aztec city a single galley departed it and sailed up the coast. They've been sitting within my borders on my coast for the last 2000 years, no cities, wounded galley in the yellow...