Smoke back in the olden days, the designs weren't as large and complex as they are now. They were created using a scythe, or sometimes burnt.
And just because someone doesnt' step forward and claim it doesn't mean it's something supernatural.

I've done some absolutely marvelous works of art using toilet paper on friends yards back in the day, and I never stepped forward :P

--All you really need is love, but a little chocolate now and then doesn't hurt-- Lucy Van Pelt