Mystery

So, I stopped at a gas station on the way home from school today to put a bit of petrol in the car (it's been running on "E" since Tuesday night.  Yes, I know.)

And the cap for my gas tank wasn't there.

My reaction was to give it a blank stare, and then to say "What?  What?  What?? ... WHAT."  Out loud, and with feeling.  Very much in the manner of David Tennant as the Doctor, in fact, although that did not occur to me until later.

Like this.

I can only assume that someone stole it, because when I tanked up last week I distinctly remember screwing it back in.  Besides, it's attached to my car -- it's not like I could drive off and leave it somewhere. 

But why would you steal a gas cap?  What practical purpose does it serve, when removed from its proper place?  (If the culprit, should there actually have been one, was trying to steal gas, he would have been sadly disappointed.  I had none to give him.)

It is an hour later, and I am still saying "What?  What??  WHAT."  I can't seem to think of any other response.