It amazes me how much dirt accumulates in the trunk, stuff like small leaves and bits of whatever. I assume it is from driving with the top down because if the stuff was getting in with the top up, I would probably have water when it rains or when the car goes through a car wash. 2010 IS250C

 

It should be from left over dirt on the roof and also in the crevice between the rear window and trunk that gets transferred inside the trunk when you put the top down.

 

A possibility. I'll have to try to remember to look for this before lowering the top.