It seems AI lands where it first took off from, if they took off from a base. Presuming fences are set correctly, it even doesn't matter if they are assigned to a carrier or not. If they took off from a carrier, they will return to the carrier, no matter where you've set them to land. I'm working on this right now cause I stepped into the same "problem" during creating a set of campaign missions. Haven't testet with additional placed airfields yet...