My car's gas door

I was pulling out of the gas station yesterday and this scruffy looking guy drives up behind me in his old beater and starts energetically gesticulating. I didn't know what to make of this and thought he was probably angry at me for some inadvertent slight. He got out of his car and walked rapidly towards mine. Oh Oh.
Then he reaches toward my car and . . . shuts my gas cap cover--once again I accidentally left it open (not the cap; the door)--gives me a thumbs-up and walks back to his car.

I don't know why--maybe because I'm getting older--I forget to close the gas door every now and then. Some kind person always alerts me or, more often, runs up to my car and closes it themselves. It's almost always someone driving an older or fairly downscale car. I have never once left a gas station with the door open. It's a small thing, but anything that provides a reason for optimism is good.