It was November 2017, and Christie’s, the venerable auction house in New York, was about to sell Salvator Mundi, the rediscovered painting by Leonardo da Vinci. They were hoping to fetch a disgusting amount of money. Croesus money. Robber baron money. The kind of money that gives an art dealers a better margin than a cartel boss.

So to drum up publicity, the folks at Christie’s launched an ad campaign with a sincerity that only Americans are capable of. They released a video of “ordinary people” – hunky men and gorgeous women who looked like they had just walked off the set of a Marvel movie – emotionally stricken by the painting’s beauty. It looked like an epidemic of Stendhal syndrome in Rockefeller Centre.

There they stood, unspeakably moved – moist eyes, hands clasped to their mouths, scraping their eyebrows off the ceiling – as if they themselves had received the blessing from Jesus Christ depicted on the canvas. “There are no words to describe the experience of being close to this painting,” said one viewer, doubtlessly unfamiliar with the phenomenon of mass hysteria. “After seeing this painting I was filled with the certainty that, one day, all human beings will be able to live together in peace.”

How those visitors choose to remember their experience of weeping in public (the cameras on their good side) probably depends on which of two shocking details they heard first. One, the painting has disappeared. Two, da Vinci might not have painted it.

But by that point, it wouldn’t have mattered to Christie’s. The auction, when it came, was so frantic that one observer likened it to a cock fight. Christie’s sold Salvator Mundi for $400 million, with an extra $50 million for commission. This was the highest price ever paid for a painting. Hallelujah!

The incredible rumours and mystery surrounding da Vinci’s lost artwork have swirled around for many years and have been given fresh oomph thanks to a gripping new documentary released by the French journalist Antoine Vitkane, which premiered this summer at Sheffield’s DocFest.