NOTHING IS MORE ordinary than a Monday morning at a Swedish bank.

People go about their business quietly, with Scandinavian efficiency. The weather outside is, more likely than not, cold and gray. But on April 22, 2013, the scene at Stockholm's Östermalmstorg branch of Skandinaviska Enskilda Banken got a jolt of color. At 10:30 am, a man in a black cap burst into the building. "This is a robbery!" he announced, using one arm to point a gun at the bankers and the other to hold out a cloth bag. "I want cash!"

If the staff was alarmed, no one much showed it. Instead, the employees calmly informed the stranger that his demands could not be met. The bank, they explained, had no cash on the premises. None in the vaults, none at the tellers' windows, none at all. When the robber looked confused, he was directed to a poster on the wall that proclaimed this a "cash-free" location. "It's true," the manager told him.