SANTA CLARA — A Santa Clara police sergeant is expected to go on trial next week on a misdemeanor charge that he exposed himself to his girlfriend’s co-worker after having sex with his sweetheart in the bathroom and back room of her workplace.

Sgt. Thomas Leipelt, 46, has been on paid administrative leave since his arrest last summer, but the case has been kept so under wraps by the Santa Clara Police Department that even Mayor Lisa Gillmor was unaware of the arrest until this newspaper asked her about it last week.

Leipelt, 46, of Santa Clara, was arrested in July after a May 15, 2015, incident at a San Jose business where he allegedly had sex with his girlfriend in a back room and later masturbated in front of a co-worker.

The case is raising questions about the public’s right to know when one of their city’s officers runs afoul of the law. Santa Clara police say they didn’t publicize the arrest since Leipelt was off-duty and San Jose police investigated, and cited the officer’s legal and privacy protections under the Police Officer Bill of Rights.

“The arrest stemmed from an incident that occurred in San Jose when he was off duty,” Lt. Kurt Clarke said in a written statement. “A press release was not provided to the media by our police department because it was a personnel matter and the criminal investigation was being investigated by another agency.”

The city administration declined to comment, citing similar reasons.

That’s not adequate to open-government advocates pushing for more transparency from police.

“This is something the public has a strong interest in knowing,” said Peter Bibring, director of police practices for the ACLU of California. “When the public discovers criminal charges pending against an officer, and that the department didn’t see fit to make that known, that undermines trust.”

Gillmor, who was recently appointed mayor and has pushed for more sunshine in city government, said she had not heard about the case until notified by this newspaper. The city declined to comment on whether the case was disclosed to council members.

“I was not aware of any of this,” she said, noting that it had not come up in any closed session meetings where personnel matters were discussed. She said little else about the incident.

“I don’t want to pass judgment on this particular case,” she said. “But moving forward, I want to see Santa Clara be more transparent.”

The police department’s public-disclosure sensibilities also came under question in January, when the Jan. 8 drowning of a 12-year-old boy led to his mother’s arrest. Police did not publicize the homicide, relegating it to a nondescript arrest-log entry until asked by this newspaper.

Bibring said Leipelt’s case highlights why his organization is backing SB 1286, a bill sponsored by State Sen. Mark Leno, D-San Francisco, that resurrects past efforts to increase public access to police disciplinary records and hearings.

Dennis Kenney, a professor at the John Jay College of Criminal Justice in New York, said the way Santa Clara police handled the case was not unusual.

“Most agencies try to handle that as a personnel matter,” Kenney said. “But once the charges were filed, it should have been made public. It’s relevant to the public because it causes one to wonder what kind of people you’re hiring.”

The incident surfaced when a woman reported to a Santa Clara internal affairs sergeant on May 26 that Leipelt had exposed his genitals to her May 15 at the business where she worked in San Jose, according to a police report. The sergeant contacted San Jose police, and sexual-assault detectives were assigned to the case May 28.

According to the police report, Leipelt entered the undisclosed business around 4:30 p.m. to meet a woman he was dating who worked there. During her lunch break, the two had sex in a back room and then a bathroom. The woman then went back to the sales floor to tend to a customer.

A few minutes later, a female co-worker of the girlfriend went to the back room to retrieve a document from a printer. She reportedly discovered Leipelt nude and masturbating, according to the police report. The co-worker told police she and Leipelt made eye contact and that Leipelt said to her, “come here,” prompting the co-worker to leave the room.

The co-worker told Leipelt’s girlfriend what she saw. The girlfriend went into the room where Leipelt was. Five to seven minutes later, both emerged from the room and apologized to the co-worker, who said she was “sick to her stomach, upset and very disgusted,” according to the police report. A few days later, the girlfriend was fired.

The co-worker, identified by police as the victim, identified Leipelt from a photo lineup presented by the San Jose detectives. When asked how she wanted to proceed with potential charges, she told detectives that “the thing that bothered her is (the suspect) had no reaction when she walked in, leading her to believe this was not the first time.”

An arrest warrant was issued and Leipelt was booked July 16, and was then released on $5,000 bail. His trial is tentatively scheduled to begin March 21. A misdemeanor conviction would not automatically disqualify Leipelt from remaining a police officer, and his continued employment would be at the discretion of the police department. A conviction would also require him to register as a sex offender in the state.

Cameron Bowman, Leipelt’s attorney, said he was not going to try the case in the press and his client “is looking forward to the opportunity to talk about what really happened.”

“I’m not sure why the DA’s office would think that a respected, well liked, and professional police officer would suddenly one day become a sex offender,” Bowman said. “That’s not what happened here and we’re confident when everything comes out, the jury will do the right thing.”

Leipelt, a 15-year department veteran, last worked in the community services unit that among its duties oversaw the D.A.R.E. youth anti-drug program.

Kenney said he believes police should have been more forthcoming about Leipelt’s charge, but advised a tempered reaction.

“It might indicate maybe boorish behavior by an officer, but it doesn’t really indicate a corrupt police department,” he said.

For Bibring, the ACLU attorney, measuring the severity of the allegation is in some ways missing the point, particularly in a national environment of heightened attention to police-community relations.

“Police repeatedly say they want the public to trust them, but they have to earn that trust by being transparent.”

Contact Robert Salonga at 408-920-5002. Follow him at Twitter.com/robertsalonga.