If the cat has previously been clean and this is a new habit, then a vet check up is a good idea. At 16 years it might be the cat has a health problem which is causing them to drink more and then pee more. It's interesting that the preferred location is the sofa because this is strongly linked to your presence and the scent will be strongly reassuring for the cat. Thus, in a back-handed compliment the cat is toiletting on the sofa as they feel safe there. Before assuming this is behavioral the cat should be checked by a vet.