For an art heist, it was a simple affair. A man ducked into a San Francisco gallery and walked out less than a minute later holding one of its best pieces: a $20,000 Salvador Dalí etching.

“I was alone at the gallery and turned my back for a minute, and when I looked it was gone,” Rasjad Hopkins, an associate director of the gallery, said on Tuesday. “I never saw the person.”

The artwork, a limited-edition hand-colored 1960s Surrealist etching titled “La Girafe en Feu,” or “The Giraffe on Fire,” had been sitting on an easel at the gallery, Dennis Rae Fine Art, when it was taken on Sunday.

Normally, the Spanish artist’s work would be tethered to the easel, Mr. Hopkins said, but on the day of the theft it was not. A security camera in the gallery was not turned on at the time.