In the Med you could use dead reckoning and when you run into the coast follow it to the nearest port.
No doubt there are methods, but did the Romans know and use them in the 3rd century? I’m not sure without doing some research but I was just guessing that if researchers believed they didn’t sail in open ocean, one would conclude that there was no evidence that they had open ocean navigation techniques. As someone upthread mentioned, perhaps they got blown off course. Then again, for all we know, they may have hugged the shore just to do port hops and hit a different brothel each night.