No one should have known about it. The painting should have been slipped silently into the back of the Fogg at a time undisclosed to the public. It should not have been opened until the proper fanfare could be mustered.

But Armand Hammer, perhaps the world's most flamboyant businessman even at the age of 77, didn't want it that way. He wasn't content with presenting "Juno", his recently purchased $3.25-million Rembrandt, without a touch of the swashbuckler.

So Hammer, active chairman of Occidental Petroleum, and his wife Francis, came to the Fogg yesterday with the painting, and the publicity and the needless security risks. And everybody had a very good time.

Hammer made it clear at the outset that he was lending Harvard the painting for a three-month show beginning Thursday only because he wanted to please his "dear friend" Seymour Slive, director of the Fogg.

"He [Slive] was disappointed that the painting was at the Metropolitan Museum all those years--this is the only thing I could do for our friendship," Hammer explained about his decision to bring the painting to the Fogg immediately after he purchased it.

But the three-month presentation of Juno - the most expensive Rembrandt ever purchased -- isn't all that Harvard stands to gain out of Hammer's presence.

As Slive ushered Hammer through the halls and bowels of the Fogg, past the museum's tremendous collection of Impressionists and its awesome restoration facilities, it became clear that Hammer may be interested in helping Harvard out in other ways as well.

For instance, after viewing the University's extensive collection of reserves--paintings that only students get to see because the Fogg doesn't have enough wall space, Hammer said he would consider giving some money towards the construction of a planned $17 million Fogg addition to house the objects.

And, considering Hammer's extensive relations with the Soviet Union--he has had dealings with Russian leaders as diverse as Lenin and Trotsky and was the motivating force behind a huge phosphorus deal with the Soviets several years ago--it didn't come as any surprise when he said he would give money to the current Russian Research Center Fund Drive.

Oohs and Aahs

But most of the fun was reserved for the Hammers as they oohed, aawed, and gee whizzed their way through the Fogg's Renoirs, Daumiers and, of course, Rembrandts. "You've got a treasure house here," Hammer told Slive. "A small Hermitage."

The Wertheim Collection of Impressionists brought out the story-teller in Hammer. After hearing Slive discourse on how Van Gogh's art was considered degenerate by the Nazis, Hammer related a story about his newly-purchased Rembrandt.

He mentioned that Juno was in the basement of a Bonn museum in 1934 when Hitler decided that all paintings that weren't Rhenish should be sold. An astute collector snatched up the painting for 900 Marks, or $214. "We got even with Hitler," Hammer said of the deal. "That's all he got out of it."

But during his hour-long tour Hammer remained the consummate businessman--even taking phone calls, one of them from Columbia's Soviet scholar and Carter adviser Zbigniew Brzezinski to arrange a meeting later that afternoon, presumably to discuss Soviet-American relations.

On the subject of those relations, Hammer recalled an encounter he once had with Trotsky, a man he used to debate about the Communist and capitalist systems.

Trotsky Offer

Trotsky, he said, told him, "You know we want you to take a concession in Russia." At the time Hammer says he was a little nonplussed, unsure about what Trotsky had in mind. Until, he says, Trotsky added, "But when the revolution comes in America, we will take it all back."

Although Hammer is far past traditional retirement age, William McSweeny, president of Occidental International, who accompanied the Hammers on their trip said, when asked whether Hammer still runs the show at Occidental, "You bet your life, he does."

Even though University Police Chief David Gorski said that Hammer's flair made it more difficult to safeguard the painting, Gorski acknowledge yesterday that the affair was all Hammer's, and he wasn't about to quarrel with a gift horse