2nd gen Prius aero visual evidence.
 

Provided by a dusty road. Check out the perfectly straight line in the dust along the back edge of the spoiler.

What's weird is the dust shadows left by two round stickers on the glass and a big one on the hatch. The glass and paint look completely clean yet something left behind attracted dust on the glass and repelled it on the paint.

That's odd. Odd is good, it makes you wonder.

When the car is clean, can you feel the difference with your fingertips?

It's either adhesive residue or an electrostatic phenomenon. Maybe both are necessary?

Don't feel anything there. Must be some kind of extremely thin residue, at least on the glass. Whatever the Honda dealer did to remove the stickers didn't get rid of everything.

I went to a car wash the other day and gave it all a good blasting with the soap then rinsed.

I'll see if it shows up again, though not planning on any dusty, dirty trips just to test it. ;)

Interesting. The line along the spoiler lip is familiar.
But hey, what's with that sticker shadow's size and shape: either this was a Transformer car or you have a former fire department vehicle.

Isopropyl Alchohol.

Optimus Prius.