The Montgomery County States Attorney's Office has informed the county's chief administrative officer that it is preparing a report indicating that derogatory information on some county employees was kept in their personnel files, according to informed sources.

When allegations of the existence of secret personnel files surfaced two years ago, top county officials including Administrative Officer William Hussman, denied that such files existed. Hussman said yesterday there had never been a system of collecting derogatory data or information unrelated to jobs about county workers and using it against them.

State's Attorney Andrew L. Senner would not comment on the matter except to say "there will be some activity in that area (personnel files) in the future." Sonner's office seized about 100 personnel files 21 months ago, according to sources. These will be returned to county officials along with the report, which reportedly has not yet been written.

The reports is expected to examing the personnel practices that allowed the derogatory material to end up in sealed brown envelopes in the county personnel files of some workers, sources said this week. The sources said this information includes details of the private lives of county employees or their relatives.

Hussman added, "I think you will find that whatever Sonner produces will deal with what happened four or five years ago, not in the last couple of years and not now."

Sonner's office seized the files in April, 1975, amid a storm of allegations that some derogatory information had been improperly gathered and filed that police officers had gathered information of some employees and that some of the secret material was being destroyed.

The allegations of secret files had surfaced in newspaper articles in the fall of 1974 in the midst of an investigation by the State's Attorney's Office of county contracting procedures.

If Sonner's report and recommendations to the county are produced as expected, it will be the first official notice that has been taken of the allegations in more than 18 months. For six months, from the fall of 1974 to the summer of 1975, newspaper reports charted the progress of Sonner's investigation into county contracts, the anonymous charges about secret files and denials by Hussman and County Executive James P. Gleason that the files existed.

The furor over the files died down about nine months after their existence had first been alleged in a story in the weekly Montgomery County Journal, but the question of whether some county personnel files had contained improper material was left unresolved.

Immediately after the first reports of the files' existence, Hussman and Gleason, in an interview with a local paper, insisted that no such files were kept.

Hussman said yesterday, "people had alleged that the county for a system that tracked down people's lives in an orderly way with an intent on the part of the government of using the information against people.

"I don't believe such a system existed," he said.

Asked if improper materials had ever ended up in the county personnel files, Hussman said, "that may or may not have happened. . . I don't know that it did. All sorts of things can get into files."

He added that, after the original allegations, the county allowed individual employees who wished to see their personnel files to go through them. "We did that in order to get out of the files what may not belong there," he said.

The only confidential material that was kept in files "of necessity," said Hussman, consisted of employees' job histories and medical reports.

The county personnel board, after an investigation, reported two years ago that its study of more than 30 sealed brown envelopes found in personnel files showed that the sealed files contained "no information which could be considered even remotely detrimemtal or defamatory."

The material that Sonner seized four months later, after reportedly receiving a tip that it was about to be destroyed, did contain material "that definitely should not have been in personnel files," Sonner said in May, 1975. Sources said last week that such material typically might include information on the arrest of an employees' spouse on morals changes.

According to recent reports from informed sources, Sooner's office found while studying the files that information sometimes passed from police reports into personnel files.

Col. Kenneth Watkins, who retired last spring after five years as head of the county police force, said yesterday, "quite a few years ago the Personnel Department would call on the Department of Police to make certain inquiries." The inquiries, he said, involved only allegations of criminal activity by county employees of noncriminal background checks of applications for "high ranking" posts in the county.

Walkins added that "I have no knowledge" that any police-gathered information ended up in personnel files. When asked if he felt that nothing improper had been done by the police force he said, "correct."

Sources have said that the alleged information-gathering activities of the Personnel Office were conducted when John P. Gaquin was director of personnel for the county. Guquin, who held that office for 21 years, committed suicide in 1972.

There has been no indication that the State's Attorneys Office has planned to bring criminal charges in connection with the probe of secret files.