It's quite common to see moisture in car's head-or tail lights. Whether water started seeping in after the light casing was cracked following an accident, or simply because it's getting old, water's getting in there, and it's a pain in the "exhaust pipe", if you catch my drift.
In my case, I suspect water started seeping in after I stupidly reversed without properly checking my blind spot. Yeah, I still cringe in shame when I think about it.
The result is nonetheless there: my rear right tail light turned into a fish tank, which annoyingly fogs up when driving at night because of the heat of the light bulbs. It was time to pour out the water and make sure it never came back.