Not flare! Cannot be flare under these varied lighting situations. It seems it's in the same spot, I would check the sensor first, it could be defective.
i have dust spots that look very similar to this, only the biggest ones are only about half that size, and there's a few of them, and they only show up on F9 or F11 or higher... still annoying, i am going to try the blower bulb thing before trying to clean it...