I work very close to this place. I walk by most days at lunch. I haven't seen anyone go in or come out. I don't think there's a back entrance, but there might be an entrance from inside the hotel (Americus), if you can even go inside there yet.
The large windows on either side of the door are covered with an opaque e film (not paper) on the lower half. They did add white rope lights around the windows and the usual red LED "OPEN" sign. Aside from that, it doesn't really stand out. If they didn't add the tacky lights, it would still look like the original Serenity Spa.