Mine has water marks inside, as I've had constant problems with the lights since they were opened to be sprayed. I'm planning on splitting them and re-sealing them over the summer. Maybe thats an idea worth considering? Maybe there is a seal problem that keeps causing it. A few days under the stairs drys them out but I still get the problem back. Going to go crazy with some black silicone this summer and clean the lenses, and just hope it fixes the problem.
I'm thinking of modifying one of the top clean caps with some metal tube, to somewhere it can vent air to keep dry. I'll let you know how I get on as this might be an idea if it keeps happening to you also.