She is among the little-noticed persons who live in Washington - squirreled away in apartments and houses, often in suburbs. Friendly, but not too friendly; liked by the neighbors, but not really known. Someone who comes and goes for years of time.

And then, suddenly, the morning newspaper arrives and there she is, staring stoically out of a front page, looking not at all like a next-door neighbor.

She is now charged, one year later by the Russians, with transporting poison used to kill an unidentified person while she was in Moscow last summer. She is not the Foreign Service officer she said she was. Official Washington sources confirm: She was a CIA agent, Martha D. (Marty) Peterson. A mystery woman. As they say in the trade , it was a apparently, an extremely good cover.

In some two dozen interviews with people who knew Marty Peterson in Russia and in her middle-class suburban neighborhood five miles from CIA headquarters, there is surprise over Peterson's second life.

"One of the great exercises while in Moscow was to try to find out who were CIA amony the F.S.O.s," says one American living in Moscow. "I can't recall anyone who ever even imagined Marty, which makes me think now, 'Boy she must have been a good one.'"

Falls Church neighbor, whose garden court townhouse faces Peterson's, said, "I just couldn't believe it. That picture doesn't look anything like her."

There was nothing convert about 33-year-old Peterson's personality - the adjectives used over and over to describe her are "wholesome, outgoing, and All-American type, good-natured, ladylike, vivacious, bullient, friendly." The kind, one Moscow co-worker said, "who could get excited about a checker game." Played a piano-organ combination at parties, waved hello in the embassy snack bar to people she had met but once, offered Falls Church neighbors rides home from the grocery store. The neighbor who sunbathed on her balcony and asked the teenager to cut the lawn.

About 5 feet 7, not fat but a bit hefty, with frosted-blond hair. Quite glamorous when she dressed up-Athletic. A green-belt in Tae Kwon do, a Korean martial art. Referred to herself as "Mrs. Peterson."

Then formally comes the stunning realization that these people virtually nothing concrete about this woman described as wholesome, friendly, outgoing, good-natured. Not where she went to school, the name of her parents, where she grew up, her maiden name, what she did before she went to Moscow.

Oh yes, there is this one thing.

Mentioned by nearly everyone - even those who say, "I barely know her at all." It was the tragedy in her life. Her husband was a Navy pilot, killed in Laos in 1973. The source is always the same. Marty Peterson. One former CIA agent knows a man who says he saw Marty and her husband together in Laos. He does not know the first name of the husband.

A computerized check of Department of Defense casualties reveals no Navy pilot by the name in any other branch, killed in action or accidentally in Southeast Asia.

The Pentagon is now checking its files to confirm preliminary investigations which indicate that he is a John Peterson who served in the Army Special Forces - from 1967 to 1969.

The alleged episode of Marty Peterson's cloak-and-dagger-espionage is generally viewed as a retaliatory responded to recent American disclosures that Soviet easedropping gear had been secreted within the U.S. embassy in Moscow. It also seems to be linked to the arrest in New Jersey of two Soviet citizens accused of espionage.

Marty Peterson seems caught up in the general heightening of tensions between Moscow and Washington. In recent years, both governments have rarely publicized each other's espionage activities, allowing the quet expulsion of apprehended spies. The Neighborhood

The tale of Marty Peterson skulking in the streets of Moscow, planting a rock containing poison capsules in a niche of a bridge, seems absurdly hizarre in the everyday world of her Fall Church home, where homeowners are busily tracking down their enemy in these hot, humid days: crabgrass.

A tall boxwood half-conceals the door to her $90,000 three-bedroom, cream-colored brick townhouse.From the front, it looks as if no one is home. Venetian blinds are tightly closed at windows and doorway. From the back courtyard, lights can be seen. Peterson returned there lastt fall on her return from Russian.

TA young, clean-cut, dark-haired man in T-shirt and dark slacks comes out to get a newspaper and says Marty is expected shortly. She never shows that night. Last week one neighbor says Marty told her she was going to vacation for two weeks. Another says she was certain she was there a few day ago. Another says she was not appeared in her back yard for two weeks. Another says she took her trash out Tuesday night. The phone has been alternately busy, off the hook, not answered or answered by a man who is always vague about Marty's return or whereabouts. The man answering the door says he is a "friend of a friend." One neighbor says. "I don't know him at all but Marty is very kind and nice. She told him to cut grass while she was gone. No, I don't know when she'll be back."

A few minutes later the dark-haired man, dressed in a seersucker jacker and slacks, tools out of Peterson's garage in a sporty white Pontiac Firebird with a red-and-blue pinstripe trim. The $7,000 car was purchased last August in Pompano Beach, Fla., by Martin D. Peterson. She paid cash.

Not bad for a woman who was supposed to have a junior-level F.S.O. job that paid no more than $13,000 a year. Peterson moved into her townhouse in 1974, lived there for about a year. "Then, one day, she came over and told us she was going to Russia for her State Department job," said one neighbor. "She seemed real excited." The Embassy, Moscow

From November 1975 to August 1977, Marty Peterson worked in several sections of the embassy in Moscow.

One bachelor who met her at the 1975 black-tie Marine ball shortly after she arrived in Moscow, remembers her as dressed all in white and "about the most attractive single woman I'd seen at the embassy up to that point. But one F.S.O. wife took me aside and explained that I shouldn't quiz her too closely about her background. That this was her first time out (at a post) since the tragic death of her husband. It seemed quite understandable that she could be protecting herself from a past painful experience." He didn't pursue her because, "I was somewhat put off hearing that, and didn't feel

"I know this sound kind of sexist, but at that time I thought of her as a naive young woman, a stranger in a mystifying world."

Her next door neighbor in her Moscow apartment house recalls her as a willing catsitter when they went out of town, but that's about all. On the phone from another Iron Curtain country, he said, "Even if I knew anything, I really couldn't say. Talking on the phone here is like talking to a tape recorder. We're monitored all the time."

One woman who said, "Marty had great sense of humor," added that there was any talk of the past or the future, or of politics. "The center of everything was our living this ghetto existence in Russia; the present was everything."

One friend, now back in the States, described himself as "very close" to Peterson" - "a simply marvelous person" - but would not talk about her. He also said he knew nothing of her background, except that her monther lived somewhere in Florida.

It seemed natural to some Foreign Service officers stationed in Russia to know little about their friends and colleagues.

"Service in Moscow is equivalent to a foxhole," said one. "There were so few places you could speak openly.Offices? Bedrooms? All are bugged. you resorted to writing on paper." 'Just a Lovely Girl'

It is also a place where a CIA agent has to live one's cover, and live it well to Divert suspicion.

Marty Peterson was a vice-counsul who interviewed potential Soviet immigrants to the United States and helped American visitors with passport problems.

And that is why one March morning in 1977, just a few months before the Russians expelled her as a spy, Peterson visited the Moscow hotel room of a distraught Washington. Mrs. Simon Tulchin, 75, recalls her distress following her husband' fatal heart attack during their tour of RUSSIA.

Tulchin initially had trouble getting anyone from the embassy to come to the hotel to expedite her departure. "But the next morning this Mrs. Peterson appeared. She canceled my husband's passport so that no one could get a hold of it and stayed about a half hour. She was just a lovely girl and a credit to the Foreign Service. Very considerate. She told me about losing her own husband. She left me with the impression she wanted to do something after her husband died, to do something for her country." The 'Edpionage Rock'

The Isvestia account of Peterson's cloak-and-dagger activities sounds like a Russian version of a Hollywood version of subversion. It was, in fact, written by Yulian Semenov, Russian's most famous spy novelist.

On July 15, the Soviet newspaper reported, "A girl, working as U.S. vice consul in Moscow, got into a car and drove to the center of the city. In a poorly lit place she changed her dress, locked the car and boarded a city bus. After changing transport several times she finally hurried to the bridge linking Luzhniki with the Lenin Hills and put an ordinary-looking stone in a loophole in the arch. It was there the vice-consul was detained."

The account went on: Embassy officials were quickly summoned, while the Russians held Peterson. The "stone," the Soviets said, was cracked open and contained a microphone, photo cameras, a large sum of money, two ampules of poison and "special instructions" on how it should be used. The espionage rock was to have been retrieved by some anonymous spy.

Further, from Isvestia: When apprehended, Peterson, "started shouting, 'I am a foreigner!' Obviously the vice-consul was shouting so loudly to warn the spy who was coming to the appointment place about the danger."

The Russians claim that Peterson had previously transmitted poison used in killing an "innocent Soviet citizen who stood in the way" of a CIA-run espionage ring. (The alleged victim and the spy who carried out the execution were not identified.)

According to the Russian version: When police questioned her about the name of the intended victim of the poison, U.S. Consul Clifford Gross advised Peterson to reply. But Peterson, Isvestia claimed, told him to "shut up." Gross then allegedly told her interrogators: "No use asking her. She is only the executor." Then, the paper said the "pretty CIA agent literally roared at him, 'Shut up!'"

Those who have served in Moscow scoff and doubt most of the details are accurate. "They are so farfetched; the Soviets will say anything," says a woman who knew Peterson in Moscow. Persons Non Grata

Shortly after the alleged incitent and her interrogation, Peterson, who had diplomatic immunity, was allowed to leave Moscow on the first available plane and was declared persona non grata.

Her co-workers heard that she was leaving Russia for "family reasons."

Back in Falls Church, Peterson has kept more to herself in the neighborhood for the past nine months. One neighbor whose daughter used to cut her lawn said, "I haven't even talked with her since she's been back."

Another grumbled: "That girl is so to herself, I don't know what all. Guys come and go there but she doesn't associate with anyone around here. I can't understand people so close-mouthed. A girl came asking questions about her last winter. Told me she was from the CIA."

There is still an official number for her at State; the person answering informs that Peterson is "on leave." There is also an official classified biography. Two lines. Her date of birth: May 27, 1945 and her year of entry into the foreign service: 1975.

And at the Russian embassy, a man with a strong accent says, "We are in the same position you are in. All we know is the Izvestia story. We know nothing more of her."

And so Marty Peterson sneaks in and out of her townhouse, highly protected by a handful of friends, hiding from the press and everyone else, a shadow in the world of Cold War spying.