Not to be gross, but I figure the trunk was carpeted, so that the moisture would condense, move downward and make for a very boggy carpet area.
The trunk isn't completely airtight (according to Tesla, it's designed so that the passenger compartment can get air from the flow over the front of the car, through the "frunk" and into an air scoop).
I have the same questions. Somehow, when I leave oranges in our little fridge (seals really well IMO), they can dry out, then there's condensation inside the fridge (which makes other vegetables get soggy).
I assume the sides of the trunk (and maybe even the inner side of the lid as well) are also carpeted.
As long as it was hot, the water from the body would keep evaporating into the air in the trunk (perhaps attracting insects, hard to say). What it would not do is go back inside the body. Apparently the body was wrapped in plastic, so some of the moisture would be inside the plastic, but again, they wouldn't have weighed the plastic - just the visible remains of the body. Here's the local news source for the body being inside a plastic bag:
A decomposing body was found in a Tesla reportedly registered to singer D4vd several days after it was abandoned and taken to a Hollywood tow yard.
www.foxla.com
Lots of little mysteries in this case.
JMO