Creepy rear windshield decal?

[ProfessorX5]

I tried to google this, and found nothing, so now I find it creepy. My X has a plain green oval sticker on the rear windshield, upper right corner. I've seen similar ones on other cars. They're always a plain solid color, no words or pictures, and always on the upper right corner of the windshield.

Anyone know what this is about? It kind of creeps me out that there's nothing about this to be found on search engines.