It looks like a optical reflection, but check your sensor for dust too.
Have you taken the photo with a polfilter on your lens or without a filter?
Some of the first 5D Mark III´s had problems with "hotpixels". Hotpixels normally appear in long exposures under low light conditions, but also appear if pixels on the sensor are dead. Take a few photos. If you see the whites in all photos, send in the camera for repair.
« Last Edit: October 16, 2012, 06:29:15 AM by M.ST »
2 x 1D X, 2 x 1Ds Mark III, 5D Mark II, 350D, some new camera and lens prototypes, almost all the latest L lenses and some Zeiss lenses, Hasselblad H5D-60 with almost all lenses, D4X, Gitzo and Manfrotto tripods, Elinchrom Ranger RX Speed System, Sekonic L-758DR and a lot of other stuf