pooping in and out of the box

My 12 year old rescued male indoor/ outdoor cat w/claws had a minor problem of pooping out the box when we first got him 8 years ago. But mostly he was fine. But in the last 6 months he has started pooping in the box and out usually on the same day. In various places in the house. Same litter in same box in same place, always clean and same food. Combination of wet and dry. Same active very friendly cat he always was. The vet did various test and prescribed some medicine for ring worm. Basically a clean bill of health.  Nothing has changed. He goes out side if it is above 20 degs. for a while every day but still poops in and out of the box.  ideas? 

clean the areas with an enzyme cleaner to remove the odor because if the odor still there he will keep going there 

So the vet said he was healthy?  I would try different kinds of litter, different litterboxes in new locations, and cat attract.