THE MARKET in Geneva expectations opened on a cautious note this week, reacting to reports of cold and cloudy weather there. At midmorning it fell three points on a bulletin that Margaret Thatcher, the British prime minister, had said publicly: "We hope that the summit will be successful." The market does not like Mrs. Thatcher as a forecaster.

Expectations are very important at this summit meeting, all the Genevatologists agree, and investors throughout the world are trying to protect themselves by hedging in expectations futures. Everybody understands all too clearly that Geneva will set the trend in expectations for months to come.

The expectations market slipped again when a Soviet official, speaking on condition of anonymity, hinted at a possible proposal to break the deadlock over space weapons by setting up a joint scientific panel. The market doesn't like anonymous Russians, and it doesn't like excessive attention to Star Wars. Apprehensive, the market cancelled its lunch and sent out for a tuna sandwich and Digel.

Then, to the market's great relief, Secretary of State George Shultz commented that it was "possible but not probable" that President Reagan and General Secretary Gorbachev would agree on specific instructions to the arms negotiators. The expectations market likes Mr. Shultz. It also likes statements that try to limit the downside if nothing much comes out of this summit meeting. As long as things aren't so bad, that will be seen as pretty good.

By mid-afternoon there were rumors of heavy buying of expectations futures in West Germany, where the government is suffering a life-threatening case of the blahs and is looking urgently to Geneva to generate some badly needed cheer among its prosperous but gloomy voters. Then came the news flash that some little kid had left a note in the house where Mr. Reagan is staying, asking him (the president) to feed his (the kid's) tropical fish. That was a warm and appealing development, the market felt, setting a good tone for the discussions of nuclear weapons and human rights.

Then the pace quickened. As the talks got under way, one Russian hinted that they might go to a third day. Then another Russian hinted that they might not. German anxieties soared. The market rose strongly to close nine points up and, weary but gratified by a hard day's work, made a dinner reservation at an expensive Italian restaurant. With the summit now actually under way, these are great days for trading in expectations.