The painting was the Saudi prince’s closely guarded secret.

During the three months of the year when Sheikh Abdul Mohsen Abdulmalik Al-Sheikh cruised the sea aboard his 246-foot super yacht, the Coral Island, the canvas hung from a wall inside the luxury boat, wired to an alarm system, reportedly part of a floating museum of artwork worth hundreds of millions of dollars the deep-pocketed Saudi royal family member kept on board. But this particular piece — “Buste de Femme (Dora Maar),” a 1938 painting by Pablo Picasso — was not for show.

“None of the owner’s friends, except for one other person, knew he owned the painting,” an investigator told the Independent in 1999. “Last year when he was entertaining guests, he had the painting removed because he didn’t want anyone to know he had it.”

In March 1999, the Coral Island was bobbing at the docks of Antibes, a resort town on the French Riviera. The yacht was scheduled to travel to Barcelona for maintenance, the Independent reported. Workmen were already splashing new layers of paint on the walls. The Picasso was taken from its usual place and wrapped up for travel to a bank vault where it would be kept while the boat was being renovated. The painting was locked away in a room on the yacht on March 6, 1999.

But when workers came to fetch the Picasso on March 11, the painting was gone. “This is one of the strangest art robberies I have come across and we still have no clues,” an investigator hired to track down the stolen canvas told the Independent.

A reward of 400,000 euros (or $450,963) was offered, and investigators kicked around the continent looking for clues to the heist. But the stolen Picasso remained off the radar — until earlier this month, when two men knocked at the Amsterdam apartment of respected art historian Arthur Brand.

Dubbed the “Indiana Jones of the art world” by the BBC, Brand is an expert whose specialty is not just the marbled halls of museums, but the back-channels and byways that circulate stolen masterpieces.

“These criminals steal art because they have seen the James Bond movie ‘Dr. No’ and they think that there are rich collectors who would like to buy a stolen painting. But these Dr. Nos do not exist,” Brand told the Independent in 2016. “Then there are organizations that do artnapping for different reasons, such as the IRA, which in the Nineties used art thefts as a kind of blackmail. It was an insurance policy so as soon as they got caught they could negotiate about returning the painting and try to get a deal. It’s the same with the Italian mafia, which is also a very huge participant in the art crime.”

Brand was able to use his connections to track down the Picasso and has since turned the $28 million painting over to an unnamed insurance company.

“There is no doubt that this is the stolen Picasso,” Dick Ellis, a former Scotland Yard art squad detective who represents the insurer, told AFP.

According to a 2016 profile in the Independent, Brand began as a collector. He dove into the art world’s underground after getting burned by investments that turned out to be forgeries. Around 2002, he started a business that centered on the authenticity of European art.

“What I do with my company is we advise collectors to prevent them from buying forgeries,” Brand told the Independent. “That’s about 70 percent of my work. The rest of the time we work with Jewish families to recover works from their collections stolen by the Nazis. But a small part of our work is recoveries of thefts, and these are the things that hit the headlines, these huge cases.”

Brand’s successes have made international headlines.

In 2015, he sniffed out a pair of bronze horses cast during the Nazi reign in Germany by one of Adolf Hitler’s favorite artists, Josef Thorak. According to the German weekly magazine Der Spiegel, the sculptures, which once flanked the entrance of the Reichstag building, were believed to have been destroyed after the Soviet army busted into Berlin in the closing days of World War II.

Posing as a wealthy Dallas-based collector, Brand helped track down the sellers, a Nazi-sympathizing family that illegally possessed the bronzes, which technically belonged to the German government. Brand’s tips led to raids throughout Germany of Nazi-era antiquities.

In 2016, Brand negotiated for the return of two famous works of art — a 1941 painting by Salvador Dali and a 1929 work by Tamara de Lempicka — that had been stolen by masked men from a Dutch museum in 2009. Together, the works were valued at more than $8.5 million.

As Brand explained to AFP, with no legitimate market for such stolen high-priced pieces, looted art switches hands repeatedly, “often being used as collateral, popping up in a drug deal here, four years later in an arms deal there.”

The Picasso lifted from the Saudi super yacht in 1999 presented a unique difficulty for the sleuth.

The painting is a portrait of Dora Maar, a French photographer and painter who was romantically linked with the artist from 1936 to 1943. Picasso never sold the painting; it was hanging in his house when he died in 1973 at the age of 91. As such, it was never publicly displayed in a museum, and few photographs of the work exist. The Saudi prince purchased it in 1980 for 4 million pounds (or $4.5 million).

According to AFP, Brand first heard about the stolen Picasso floating through underground art channels in 2015. He put word out to his sources that he would like to help return the painting to the legitimate art world. He eventually learned the painting had switched hands at least 10 times since it was taken.

Then, earlier this month, he got a call.

“Two representatives of a Dutch businessman contacted me, saying their client had the painting. He was at his wits’ end,” he told AFP, while staying quiet on the identity of the businessman. “He thought the Picasso was part of a legitimate deal. It turns out the deal was legitimate — the method of payment was not.”

Last week, the two men showed up at Brand’s door in Amsterdam with the painting — now valued at $28 million — wrapped in two garbage bags.

Brand accepted the piece. The next day, an expert from the Pace Gallery in New York flew in to verify the painting’s authenticity. The insurance company will decide whether to return the painting to the Saudi prince, the BBC reported.

But before that, on the night he received the stolen art, Brand could not help taking a kind of victory lap.

“I hung the Picasso on my wall for a night,” he told AFP, “thereby making my apartment one of the most expensive in Amsterdam for a day.”