The potatoes were stored improperly for them to last that long after baking. Also, was there any chance that the potatoes were pierced with a fork to release steam or cut before baking? Those exposed surfaces, especially when wrapped tightly in foil and heated will become oxidized and turn brown or gray. Was it just the area right under the skin that turned brown? If so, they were cooked too long. This can also happen when baking a potato in foil, the skin on the bottom side especially, turns brown from over baking. Next time you might try cooking the potatoes at 185 degrees F as the potato wrapped in foil, will continue to bake.