Deborah Fitzjohn was in a talkative, happy mood when she had dinner with her grandmother almost four weeks ago. "She was," according to the private detective who has been hired to find her, "happy as a lark."

The 25-year-old left the Centreville condominium where her grandmother lives and went home about 6:30 p.m. to her own two-bedroom condominium, which is just around the corner in the Newgate Apartment complex.

She was last seen two hours later by her grandmother. She drove off on Jan. 27, a Friday night, in her 1977 Subaru station wagon. As she drove past her grandmother's kitchen window, she waved goodbye.

The car was found the next day in the parking lot of a country-western bar called Hunter's Lodge on Lee Highway, according to police, who said it bore no signs of violence.

Deborah Fitzjohn's telephone is answered now by a recorded voice that says her calls are being taken at another number. It is the number of Milda Fitzjohn, the grandmother. No one has called asking for Deborah.

Mrs. Fitzjohn, who adopted Deborah and raised her after the girl's parents separated, also hired the private detective.

For the past 27 days, the 63-year-old woman says, she has slept fitfully. She says she has been getting up in the middle of the night to sift through Deborah's things, looking for a reason why she disappeared.

The police, the private detective, Deborah's best friend and Mrs. Fitzjohn have come up with no reason why the young woman should disappear. Deborah left without packing. She never carried more than $10 cash with her, preferring to use checks or credit cards, according to her grandmother. In the last 27 days she apparently has cashed no checks and has not used her credit cards.

Deborah disappeared two days after finding out that her employer, Texaco, had agreed to pay her $653 tuition for courses in public speaking and general math at George Washington University. By all accounts, the young woman, who her friends say always talked things over when she was moody, was happy the day she disappeared.

James Wilt, the private detective, said, "She had everything in the world going for her. From the pure facts, I'd say she disappeared under suspicious circumstances."

But other than the fact of Deborah's disappearance, there are no suspicious circumstances. Deborah's condominium was undisturbed. Friends and police say she not involved in a love affair. She had been married for about a year, but that was five years ago and she had had no contact with her former husband since, according to her grandmother.

While the private detective talks to neighbors and people at Hunter's Lodge who may have seen Deborah that Friday night, the grandmother can do nothing but wait.

"The suspense of not knowing is getting worse every day," she says. Her husband, Herman, 74, a retired Army staff sergeant, has told her that night she should decide either to sleep or not go to bed at all.

It was Mrs. Fitzjohn who found her granddaughter's car. She said she was driving Saturday, Jan. 28, on Lee Highway, on her way to visit a sick aunt at Commonwealth Doctor's Hospital, when she spotted it. She called police that night.

At first, she says, the police left her "out in the cold." They told her that young adults sometimes just run off. They told her that Deborah had a right to privacy. The police seemed much more cooperative after she h'red a private detective, she says.

Police issued a missing person's report to the news media yesterday.

Mrs. Fitzjohn's second adopted daughter, Barbara, who was Deborah's younger sister, was killed in a car accident in Fairfax County in 1971.

The grandmother was called to Fairfax Hospital the night of the accident and told of the 15-year-old girl's death.

The disappearance of Deborah, says Mrs. Fitzjohn with tears welling in her eyes, forces her to ask a question of herself. "It comes to me, can I go through this again?"