During the past eight months, valuable jewelry and cash have been quietly disappearing from one apartment after another at the Watergate South. The management installed deadbolt locks on the doors, but some residents of the luxurious apartment complex were too busy rushing to this island or that resort to remember to lock the locks.

Over Labor Day weekend, District of Columbia police planted a team of 18 dectectives in Watergate apartments in a game of burglar roulette, hoping they had staked out the right apartments. They baited their traps with newspapers piled up outside the doors to make it appear that no one was at home.

At 2:30 p.m. Saturday, detectives Larry Noyes and Sally Lowndes, who had been on duty for more than six hours inside one apartment, heard the sound of a playing card slipping a door lock.

Lowndes and Noyes put down the detective thrillers they were reading and froze. Slowly the door swung open, and there he stood: a nattily attired man in a blue blazer, gray slacks and a white silk shirt. He held under his arm a number of newspapers, looking for all the world like a returning resident of the apartment.

"Stand where you are. We're the police," said Noyes, as he held a gun at the suspect's face. He and Lowndes grabbed the man and arrested him.

"Don't hurt me. I'm not armed! I'm an old man," Lowndes quoted the suspect as saying.

Police identified the man as William Joseph Lee of New York City. According to his driver's license, Lee is 71 years old.

Lee was charged with burglary and held in D.C. jail in lieu of $10,000 bond. He was questioned by police about some of the 30 unsolved burglaries at the Watergate.

"At first I though he might be a friend of the owner of the apartment," Lowndes said. "You have to be careful who you jump on." But then, she said, she saw his glasses and realized that he fit the description of a man seen by a Watergate janitor on July 4. And she and Noyes jumped.

"He's a good burglar," the officer said of the old man. "He had a necktie in his hand he was going to use to wine the prints off. He was in good shape and he didn't appear to be senile."

Lowndes said that Lee said he has two months to go on probation for a burglary committed in New Jersey, and has also been convicted in New York and Massachusetts.

She said that when she asked Lee how long he'd been coming to the Watergate, he asked, "When was it built?" When she told him, she reported he said, "Since then," although he denied having broken into any apartments in the complex.

Police reported that Lee's car, a 1973 Plymouth, was parked near the Watergate. Lowndes said Lee told her he had arrived in Washington about 10 minutes prior to his arrest, had followed a group of Watergate residents through the lobby and begun looking for likely apartments.

Detective Lowndes said Lee had a white piece of paper in his pocket on which were written the numbers of five other apartments with newspapers piled in front of their doors.

"I only take what I can carry in my pockets," the officer quoted Lee as saying, "money and jewelry. If I had scored on this one I would have gone home."