hello victor, yes most likely came from AI "lost" units - like crusaders that lose to a rebellion and have nowhere to retreat.

In my experience, merc units with homelands are less frequent relative to the amount of provinces they are available. The same goes with cultural groups. So say a unit that is both catholic and orthodox and muslim will be much more frequent than a unit that is only orthodox as it will be "available" from less provinces (orthodox provinces are less than catholic ones). Genoese Sailors are genoa only iirc in vanilla so possible to have them but less frequent.