What happens to art work once it is created?

We were intrigued by news that someone has discovered what is believed to be an early Andy Warhol drawing.  BBC News reports that British tourist and collector Andy Fields found the work at a garage sale in Los Vegas and bought it for $5.00.  It's estimated value is over one million dollars.

The sketch is of 1930's singer Rudy Vallee that Warhol drew when he was nine or ten.

Although the article is short, we are curious as to why someone in Los Vegas would have an early Warhol drawing.  What happens to art work that is not recorded by a museum or other archivist?  How much art, especially works on paper, has been lost or thrown away?

Early Warhol drawing

Warhol's Marilyn 1962

Warhol's Liz, 1965

Pic of Warhol at studio 54

This week, another item was discovered that is believed to be ancient pottery that was dropped off at a Goodwill donation trailer in western New York.  A note stuffed inside the pottery said it had been found in a burial mound in Oklahoma in 1970.  The pottery could be more than 1,000 years old.  Goodwill says the pottery is being given to the Caddo Indian Nation.