For the first time that anyone could recall, Thomas O'Leary, confessed burglar and police informer, has missed a court appearance - his sentencing yesterday.

The Montgomery County courtroom was crowded with many of the poeple whose homes he had robbed, reporters and both the police officer who arrested him last summer for 26 burglaries and the police officer with whom he worked most closely for seven years putting other criminals behind bars.

At 2:20 p.m., 50 minutes after O'Leary's scheduled appearance in Circuit Court, Assistant State's Attorney Paul Kemp noted for the record, "It appears he has added a new notch to his belt."

O'Leary, 32, pleaded guilty in January to four burglaries of affluent suburban homes and Kemp was prepared to seek the maximum penalty of 40 years from Judge David L. Cahoon.

Unbeknownst to his victims, O'Leary had been living a double life, helping society in his police work even as he selectively stole from people over the years.

As his sentencing date approached, O'Leary chose to disclose his police work in a lengthy interview with The Washington Post. He expected a long prison sentence and seemed resigned to it. He was free on a $45,000 bond, and he said he hand no intention of fleeing.

He was living with a friend in Silver Spring, but he spent. Tuesday night with his wife and children at another location.

Yesterday morning O'Leary's lawyer was in court to ask that the sentencing be postponed to allow time to obtain outside pyschiatric evaluations. The motion was argued and denied.

O'Leary was scheduled to be in the Rockville office of his attorney, Barry H. Helfand, at 11 a.m. to prepare for sentencing. "He had kept every appointment he's ever had with me," Helfand said, but not this one.

At 1 p.m., Helfand said, he received a phone call from O'Leary's wife. She, O'Leary and his father had gone out for breakfast, "what everyone knew was going to be their last breakfast together," he said.

Then, O'Leary and his father had stopped briefly at his friend's home and then at a Dunkin Donuts in Wheaton. While his father sat in the car, the story went, Thomas O'Leary kept on foot."I've heard nothing more," the lawyer said.

O'Leary's failure to appear was a letdown for several victims who were prepared to testify at the sentencing about how the burglaries had affected them. One woman told a reporter how he had taken "my mother's wedding ring" and other items of great sentimental as well as monetary value. The woman said she and her husband had donated $1,000 to the construction of a new neighborhood gymnasium, and they had little sympathy for O'Leary's argument that society owed him a debt for his police work.

In others quarters there was some concern over his safety now that his role as an informer has been made public. "The consensus," said a police spokesperson, "is that he's streetwise enough not to get himself shot."

"I thought he had himself psyched up to turn himself in," said Lt. Jim Elkins, his closest police contact. "I don't expect to hear anything from him, but I'd like to be pleasantly surprised."