My guess is that this is the result of thermal cycling over time.
In a car, the temperature rises and falls with the weather conditions. When it cools down at night, the flashlight will lag the surrounding air and therefore remain warmer than the surrounding environment. When things warm up the next day, the light will lag the surrounding air and remain cooler than the surrounding environment. Unless the temperature stays constant for a while and the light gets a chance to come into thermal equilibrium, the light will always lag its surroundings.
The problem comes when you open your boot lid while the light is colder than the air outside, because then you get condensation on the light from the warm air. If it's heavy enough, it'll trickle down the light onto the carpet in your car boot, which then becomes a reservoir to keep the lowest part of the light damp.
It wouldn't be a problem if the anodising was perfect, because there wouldn't be any exposed metal to corrode. Unfortunately, the slightest defect in the anodising gives corrosion a place to start, so it can work sideways under the adjacent anodising and expand.
I've never had it happen to a flashlight of mine, but I've seen it happen to aluminium items I was keeping in my garage. I don't keep canned drinks in the garage anymore... the eventual leaks were impressively messy.
To protect an emergency light during long term storage in the car, I'd suggest putting it in a freezer bag, squeezing as much air out as possible and closing the bag up. Same goes for spare cells. If you have some desiccant sachets to hand, chuck one or two of those in for good measure.
I've never actually kept a light in a car, because I always have lights on my person, so I rely on those instead.
