UPDATE: NBC's Alan Boyle has written a fascinating article about another theory as to what the mysterious Mars light is. Once again Curiosity has thrown us a curve ball -- one explanation revealed by a NASA scientist is that the light may be caused by the shiny surface of a rock as two Navcam images taken at different times appear to feature the same light.
"One possibility is that the light is the glint from a rock surface reflecting the sun," Justin Maki, imaging scientist at NASA's Jet Propulsion Laboratory, told Boyle. "When these images were taken each day, the sun was in the same direction as the bright spot, west-northwest from the rover, and relatively low in the sky."
Another explanation could be a spot of light leaking into the camera's housing through a vent hole. "We think it's either a vent-hole light leak or a glinty rock," said Maki.
He's not ruling out the cosmic ray explanation (below), but it is very curious that a similar spot of light should be present in two separate images, taken on two different days from the same instrument. But, no, it's still not aliens. Stay tuned for updates...