Sen. Charles Percy, prospective chairman of the Senate Foreign Relations Committee, emerged from his recent trip to the Soviet Union on cloud nine. He was heartened by what the Soviets told him, satisfied he had delivered just the right messages, glad to have done Ronald Reagan a favor.

He had left the Soviets reassured, he thought, that "they will be talking next year to people with a consistent policy."

Whereupon, reports of his talks were leaked (presumably by somebody on the Reagan transition team). They had Percy actually seeming to offer the Russians a formula to resolve the Arab-Israeli conflict by creating an independent Palestinian state, with PLO Chief Yasser Arafat as the top man. Percy, of course, denied it. His ample record on that issue thoroughly belies it. Recriminations flew.

Welcome, you might say, to the Reagan high command, to the wonderful world of Washington. And that's about all there is worth saying about the big brouhaha over the Palestinian issue.

Except this: it has managed to obscure much more interesting aspects of the Percy mission to Moscow. At least, that's the impression I got from interviewing the senator the other day.

Just to begin with, nothing much seems to have been made of what Percy apparently took to be a "commitment" from Soviet President Leonid Brezhnev that the Soviets would not use their military presence in Afghanistan as a jumping-off point to grab control of Persian Gulf oil resources, or to move south in search of warm-water ports.

"I told Brezhnev that if there was a threat to the oil in the Persian Gulf there would be the greatest arms race in the history of the world," Percy recounted. Percy then told the Soviet leader, "We did pretty well building up arms in World War II and our industrial base was crude then compared to what we have today -- come and see what we can do now.

At which point, Brezhnev "looked me in the eye, and said, 'We won't block your oil -- that's not why we're there.'"

The same reassurances were not forthcoming on Soviet intentions in Poland. Brezhnev promised him there would be "no surprise" for the incoming administration, Percy says, but it was clear to him that Brezhnev was referring to "invasions" in the Third World -- that a Soviet intervention in Poland was not to be included in the category of "no surprises."

Nor were any of the Soviet leaders very precise about the future course of arms negotiations. But Percy came away with the strong impression (contrary to other fragmentary leaks from the cabled reports of American Ambassador to Moscow Thomas J. Watson Jr.) that the Soviets, on balance, would welcome a new American initiative to revive the SALT process.

Percy envisages an American proposal somehow combining those elements of SALT II to which the Reagan administration has no objection with some 23 "reservations" that were proposed in past Senate debate, and whatever the new administration decides to add.

Percy's readings of Soviet intentions are not likely to be shared by a lot of Reagan advisers. He is at odds with Reagan, as well, on the president-elect's skepticism about continuing the U.S. grain embargo. But Percy returned with what he thinks is convincing evidence that the embargo is hurting the Soviets badly. He has since urged Reagan not to lift it without careful consideration, arguing, "Why should we give it away like we did the neutron bomb and the B1 without getting anything back?"

The senator insists he is under no illusions; he's aware the "signals" he was getting are of the sort routinely tramsmitted to new American administrations. But the Soviets, he figures, had to be aware that they were signaling a significant audience.

Percy had touched base carefully in advance with President Carter, with Reagan and transition foreign policy aide Richard Allen, and with former presidents Ford and Nixon. On his first stop after leaving Moscow, he wasted no time reporting in not only to Reagan but to Britain's foreign minister, Lord Carrington, West Germany's Chancellor Helmut Schmidt, the Italians and the French.

Percy is still smoldering about what he considers "malicious" undercutting at the hands of someone in the Reagan camp. But he is nothing if not irrepressible, and buoyed all the more by his Moscow experience. Independent in his thinking, his votes on the Panama Canal Treaty and F15 sales to Saudi Arabia put him sharply at odds with Reagan policy on two key issues, at least.

Percy's new position, in short, shapes up increasingly as a force -- or counterforce -- that the hard-liners in the Reagan entourage are going to have to reckon with.