IMHO - condensation. If the camera was taken from a warm car into cold air, that might cause condensation on the inner surface of the lens. Impossible to wipe clean and as it dispersed it would no longer appear on later photos.
Yup - deffo misture somewhere on the lens - I've got hundreds like it! :^(
Peter may well be right about why it's not visible on other pix but the other thing that affects it is the angle the light's coming from, the contrast between it & the part of the pic it appears on etc, etc.