In February the 6-foot-3 former intelligence officer was quietly dispatched to Argentina, Brazil, Chile, Mexico, Panama and Venezuela. In mid-March he was spotted in Morocco. A few days ago he was believed to be in Western Europe.

America's most experienced secret emissary, retired Lt. Gen. Vernon A. Walters, last night came out of anonymity by joining Secretary of State Alexander M. Haig Jr. on his mission to the Middle East.

Since it would be impossible to hide the tall Walters aboard the secretary of state's airplane, it is acknowledged for the first time that he has been selected as a senior adviser who will act as roving ambassador for special missions.

State Department sources denied a waggish report that Walters, a phenomenal linguist who is reputed to be fluent in eight languages and has been an official translator in French, Spanish, Italian and German, has been hired to translate "Haigspeak" into English.

Walters' relationship with Haig goes back to the Nixon administration, when Haig was deputy to Henry A. Kissinger on the National Security Council staff, and Walters was Kissinger's designated secret channel, through Paris, to Chinese and North Vietnamese officials.

In his memoirs, Silent Missions, Walters referred to Haig as "Kissinger's NATO commander" and "one of the most brilliant and able officers I have known in 35 years of military service."

Haig is by no means the most prominent figure Walters has served. During World War II, Walters entered Rome in 1944 as an aide to Gen. Mark Clark. Afterward, he interpreted for President Truman in Rio in 1947, and accompanied Truman to his troubled meeting with Gen. Douglas MacArthur on Wake Island in 1950.

Walters interpreted for Averell Harriman and Iranian Prime Minister Mohammed Mossadegh in 1951, and was with President Eisenhower at a series of summit conferences from Geneva to White Sulphur Springs.

One of his closet relationships was with Richard M. Nixon, whom he accompanied to Caracas, where the then-vice president was besieged by a mob in 1958.

In May of 1971 Nixon appointed Walters deputy director of the CIA under Director Richard Helms. The Watergate break-in took place the following month, embroiling Walters in one of the most controversial and most publicized episodes of his career.

At the behest of H. R. (Bob) Haldeman, Walters visited FBI Director L. Patrick Gray III and told him that continued investigation of the Watergate case might expose CIA assets in Mexico, in effect waving the FBI off the case.

Walters wrote in his memoirs that even though he knew of no CIA assets being compromised, "It simply did not occur to me that the chief of staff to the president might be asking me to do something that was illegal or wrong."

A few days later, after looking into the matter, Walters told White House counsel John Dean that there were no CIA assets in Mexico that might be endangered by the Watergate investigation.

Walters resigned from the CIA and retired from the Army in July 1976, receiving his third Distinguished Service Medal at that time. Last year he served on the foreign policy advisory committee of Ronald Reagan's presidential campaign.

When the Reagan administration decided in February to "draw the line" against Cuban-backed insurgency in El Salvador, Walters was sent to brief the leaderrs of Latin American countries.

Walters went to North Africa to receive a message from King Hassan of Morocco, but the purpose of his reported travels in Europe is not known.