I had some loose papers in the boot, with the cover off. I rolled down all the windows to help cool it off (AC just isn't enough when it's above 100F) got up to about 40-50mph and the papers got sucked up and out the windows. So keep that cover on or secure those loose items.

After about 5000 miles I'm still learning something new about this Audi!

 

I have had the same problem with the hats that are on the rear deck lid. Didn't lose the had, but it was close.