In my area, at 12 a child can be left alone. While I probably wouldn't leave my child for a whole weekend just because he/she may sit around and do nothing but watch tv the whole time, that is the legal age hear kids can be left alone (and babysit other kids as well). I would have no problem leaving a kid home before and after school at that age though, or for evening here and there. When I was a kid at that age I took the public bus and went shopping on my own with friends.
So I'll agree that possibly the smell wouldn't be a CPS issue, but sometimes abuse does take that form. I would never start with CPS though unless I saw real obvious abuse as lots of things go on that I just wouldn't be privy too and I would never assume abuse first.
ETA: I don't wash my hair everyday and I don't get it wet as that would defeat the purpose of not washing, having your hair stay styled. However, I also don't use hair products like mouse, gel, hairspray generally so the hair is still soft and nice the second day (actually day 2 is better hair than day 1). I workout and sweat in my hair most days and I just let that dry and still wash only every second day, the odd time on the third day. If I thought the roots looked greasy I have a spray dry shampoo that absorbs the oil. If I lived in a very humid area, I maybe would still wash daily.