The absent guest at the Brezhnev-Schmidt summit which began Nov. 22 in Bonn is one Ronald Reagan. Every whisper during Soviet President Leonid Brezhnev's first visit to a Western capital since the Soviet invasion of Afghanistan will be amplified in the White House. And almost as soon as Brezhnev leaves the side of Chancellor Helmut Schmidt, a top West German career diplomat will hop onto a plane to give Washington a blow-by-blow account of the exchange.
Ranking West German and American diplomats insist that there is no concern about Brezhnev's visit at the ''top level'' of American and other allied governments. The close Reagan-Schmidt harmony following the American President's maiden foreign policy speech Nov. 18 - the ''zero option'' speech - would confirm this.
The diplomats do admit, however, that the Brezhnev visit, which ends Nov. 25, excites some concern below the top allied foreign and defense ministries.
The fear of some in the lower levels is that Moscow might be able to use the trip to bestow legitimacy on the Soviet occupation of Afghanistan - or at least to induce a business-as-usual complacency toward it. The further fear is that Brezhnev might be able to exploit American-European differences on arms and detente issues, and help Western European protesters block new NATO missile deployments without any Soviet concessions at all.
In this context the key questions in the Moscow-Bonn-Washington triangle come down to two: How energetically is the Kremlin wooing West Germany - the heart and prize of all of Western Europe - away from the United States? And how tempted are the West Germans by the courtship?
To judge by the meagerness of Soviet offers to the West German government, the answer to both questions must be: not very. Moscow is holding out to Bonn no hope whatsoever of German reunification, nor even any hope of a symbolic resumption of substantial emigration of ethnic Germans from the Soviet Union. Nor, apparently, will Brezhnev give the West Germans any gesture of European nuclear arms restraint before US-Soviet negotiations on this issue open next week.
One possible reason for the Soviet miserliness toward Bonn is reassuring to Washington. A second may be less so.
The comforting reason is that the Russians, who suffered two German occupations within memory, fear German reunification - or even just West German dominance in Western Europe - rather more than the Americans do. Not since Stalin's postwar attempt to avert West German rearmament by dangling the prospect of a neutral reunified Germany before Chancellor Konrad Adenauer has Moscow flirted with this idea.
Moreover, even without the specter of a reunified Germany the Russians have deemed continued US military presence in Western Europe a lesser evil than West Germandomination of a de-Americanized Europe. Clear proof of Moscow's preference came in the mid-'70s, when the Soviets sabotaged the Mansfield amendment for unilateral withdrawal of US troops from Europe by suddenly agreeing to multilateral force reduction talks in Europe.
Presumably, then, the Russians would be more horrified than anyone else if West Germany and the US actually split so far apart that Washington retreated across the Atlantic (as some Reagan administration thinkers reportedly dream of doing) to a fortress America.
A second possible reason for Soviet parsimony toward Bonn would be less reassuring to Washington. This is the possibility that Moscow may just be waiting for the West Germans to fall into its lap. The State Department considers such a development highly improbable; even if the coming arms control talks break down and a revolt in the ruling West German Social Democratic Party forces Chancellor Schmidt to abandon the planned new NATO nuclear missiles, the only result would be Schmidt's resignation, and a tougher line by a new conservative government in Bonn.
Nonetheless, concern about just such a scenario does arouse dark suspicions of West Germany among some members of the Reagan administration. This concern was epitomized in the recent 11th-hour European tour by an American delegation commissioned to offer Western Europe enough alternative energy to avert the impending European contract for Soviet gas supplies.
West Germany contends that its own 6 percent overall energy dependence on the Soviet Union after conclusion of this multibillion-dollar deal would not make it vulnerable to blackmail. And it argues -in rejecting not only American criticism , but also the Soviet request to make the deal's signing the centerpiece of the Brezhnev visit - that this is simply a multilateral commercial deal. It is neither bilateral nor political (and if the Bonn government has its way the final signing will take place in Dusseldorf rather than Bonn and may not even coincide with the Brezhnev visit).
Part of Washington - including, the West Germans believe, President Reagan - accepts Bonn's bona fide in playing host to Brezhnev and pressing ahead with Soviet-European economic cooperation. Part of Washington remains skeptical. Something of a question mark still hangs over the empty chair at the Schmidt-Brezhnev table.