Personally, I'd just call my dealer and be done with it. Backlight assemblies have been bad with RX-8's too (I had one myself). An airtight seal is very important, hard to create yourself. Just demand that they fix it, you shouldn't have these issues already.
Anything is possible. The LED bulb was put in months ago and I never noticed anything until a few days ago. Nothing in the trunk was wet either. Regardless, the dealer ordered a new assembly this morning and is replacing it tomorrow under warranty.
Update: in case anyone was wondering what happened.....I took it to the dealer today thinking they would replace the light. Service advisor called a few hours later to tell me that the Mazda tech assistance center told them before they replace it they should remove it, clean the water out and dry it, then reinstall. If it happens again they'll replace. So guess what, I got home and checked the light again and there was STILL some moisture in there. I found the pages in the service manual on how to remove the light. So later this week I'll take it off myself and dry it out properly. Pretty much wasted my time other than them updating my software.
If the rear lamps are anything like the fronts, there's an acceptable level of moisture buildup that Mazda head office has set in place before replacing with warranty. My headlight eventually looked like a condensor before they were willing to replace it.