if you look at the rear shelf that pops up when you put the top down
i get a lot of stuff that goes in through the small openings around it
leaves fluff etc
you probably smoke (pretty safe assumption)
and the wind blows it along the back
into the boot
or
if you have the top up
at the tiny rear shelf that is stuck to the roof
they are small vents to extract old air out of the car
if you ever open a window
the wind could blow the ashes into the boot through them
case closed