I have seen this before if a contractor was short of wire to one leg. 2 wire contains a black, and white, and a ground, 3 wire contains a black, white, red, and ground. Sometimes if a contractor was short of 2 wire, he/she might sub in 3 wire and tie back the red. Like you stated, I would test the red and if hot, leave it alone, if not hot, also leave it alone.
Some funny things show up when going into these electrical boxes.