Yeah looks like its on the lens as its not in focus. If it were on the sensor it would be in focus. Dirt and dust on the sensor show up on photos that have been taken at slower shutter speeds (the blurred rotors) because the depth of field is much greater than those taken at higher shutter speeds .
It's a pain when being all dangerous and hand holding panning shots of aircraft at really slow shutter speeds. Every bit if microscopic dust shows up.
(Ill assume you know what DOF is? Anyhoo, it's the amount of the subject that's sharp in front of and behind the point of focus, just in case!)
