It looks like sun or heat burn to me. Also the plants leaves look a little wrinkly like the plant is not getting enough water. Even if there is not direct sun, if there is a lot of heat the plant can still burn. Once damage occurs you can't do much about it other than wait for new leaves and prevent further damage.
I had this happen to a couple of my plants this summer when we had temperatures over 100 degrees. This is unusual where I live. The heat from the fluorescent lights and the hot temperatures combined and gave a couple nasty burns to my Phal. pallens. Fortunately the plant is a weed and didn't skip a beat-it looks ugly, though.