I once heard some speculation that this could also be caused by x-ray fogging from airports. I was wondering if this is possibly another avenue for the printing to be marking the film?
My thinking is that it is not the case, because unless the ink has properties that are reflective or absorptive to x-rays, the ink areas would look exactly the same as the paper areas on the film. While I suppose it is possible that the ink has a distinctive X-ray characteristic, I would think that the manufacturers would avoid this if at all possible...
Anyone know if there is any truth to x-ray fogging of this manner?