Well, just when you think you've seen everything you pull up at a stop light behind this!
I'm kinda thinking this is a little ol lady prank, and the driver of this car doesn't even know their ASU tire cover got swapped while they were in getting groceries. 
I'm wondering what one could use in place of regular yarn, which wouldn't stretch when wet, to make one. Also, one could fashion one particular to Scamp by using graph paper to design it as you could have regular crochet in the center of a trailer and block the letters for Scamp. I love crochet and while I might be a little ol lady at 60, I wasn't when I started 43 years ago.
Granny crochet? Not just for grannies!
Cathy
First thing that came to mind was they can't go through a car wash, which is along the lines you mention of the yarn getting wet.
So this could be a younger crochet gang going around the country swapping tire covers.
Our Trailers:
2015 19 Escape
Buying or Selling Molded FG Trailers:
Fiberglass-RV-4Sale
We had a tire cover disappear and no one replaced it with anything. While I would not want to mention a particular area of the country or states, there are some where the RV park choices aren't the greatest and things can grow "legs".
But, can you imagine, that your elderly grandma made you this for your vehicle, presented it with a great big smile and a hug.......................
Cathy