Since the cameras aren't sync'd to the power line, the odds of them being able to exactly math the grid frequency is zilch. My best guess if that you'd see a slow fading in and out.
It is absolutely the lights. I have seen lights do the slow fade in and out like the
Christmas Thread above. I have seen cars go by on my LPR cam that look like the headlights are off and they aren't when seen with my overview cam. I have seen it look like random off and on like they are flashing their lights.
Generally the cheaper the bulb or the power supply, the more likely a camera with a faster shutter is likely to catch this phenomena that our eyes can't see.
It is why if you select anti-flicker at 60Hz it limits the fastest shutter speed to 1/120 and at 50Hz it limits the shutter to 1/100 - any shutter speed faster and it can catch the weird cycle/frequency/modulation/whatever electrical folks call it.
With higher quality bulbs and power supply, I have been able to do odd shutter speeds to minimize that phenomena. Like if I run my LPR at 1/867 (1.15ms) instead of 1/1000, the car that looks like the headlights are off can be seen.
Now if the image is more of a wave going thru the whole image, that is more of the firmware trying to deal with trying to run the camera either with not enough light or a too fast of a shutter. Adding more light or slowing the shutter can eliminate it. Some folks have had success changing the saturation as well to minimize that wave.