For much of Tuesday, June 2, the second floor of Sotheby’s headquarters at Manhattan’s Breuer Building was off limits.

Security guards turned away employees hoping to access the floor, which, when not used as a traditional gallery, is where the auction house stages its biggest and most closely watched auctions—including the $236 million Gustav Klimt painting that last year broke the record for any work of modern art sold at auction. According to sources familiar with the matter, even senior staff were left wondering what exactly was going on upstairs.

The answer, it turns out, was a Jackson Pollock.

According to multiple sources, Sotheby’s had quietly organized a private auction for Number 19, 1951, a monumental Pollock owned by Pace Gallery founder Arne Glimcher. Measuring nearly five feet tall and four feet wide, the muscular oil-and-enamel work is filled with thick ropes of black paint coiling around each other before colliding into bold abstractions. The asking price, I’m told, was $50 million.

The sale was conducted with an unusual degree of secrecy. Oliver Barker, Sotheby’s chairman for Europe and the auction house’s star auctioneer, was flown in from London for the event despite the firm’s summer sales season fast approaching across the pond later this month. Barker was reportedly spotted walking around Midtown on Tuesday afternoon—a fact that surprised people just a few blocks away at the Breuer Building, who expected him to be in London.