STANFORD — Stanford University police logged 25 rapes and more than a dozen other sex offenses in 2015, but the campus issued a crime alert for just one of them — a groping incident in the student union.

Its students got more warnings about mountain-lion sightings than about a problem drawing unprecedented attention at campuses everywhere.

But a Bay Area News Group analysis of 2015 crime statistics and alerts for five major Bay Area campuses shows it is not unusual for colleges to keep quiet about sexual assaults, even as they warn students about other crimes.

At San Jose State, San Francisco State, Cal State East Bay and UC Berkeley that year, campus police informed students of less than 30 percent of sex offenses on or near their campuses.

Cal State East Bay did not warn students about any of the six rape and sexual battery reports it received in 2015, according to data provided by the department. San Francisco State and San Jose State each warned of groping incidents by strangers.

“Think of how the survivor feels having the campus say, `This doesn’t present a danger to anyone so we’re not going to put out an alert,’ ” said Marisa McConnell, a UC Berkeley student and advocate for sexual assault prevention.

A federal campus safety law, the Clery Act, requires colleges to collect and report annual crime statistics and to issue email or text alerts about certain crimes, from robbery to rape, when they pose a threat to the campus.

But interpreting what constitutes a threat is up to campus authorities.

In two recent cases — alleged sexual assaults last month by a San Jose State water polo player and another incident involving a Stanford graduate student — no alerts were issued, raising concerns among students why the reports were kept quiet.

Stanford did warn students last month about a student raped in a campus dorm, and UC Berkeley last week sent alerts about two sexual assaults that reportedly happened at fraternity parties.

However, as data from the most recent annual crime reports show, alerts tend to be the exception, not the rule.

San Jose State’s new president, Mary Papazian, said the campus followed federal guidelines in the water polo case but that the school would review its protocols. Campus police would not speculate about what the revisions might be, or whether they would lead to more information.

One of the student-athletes at San Jose State who reported the water polo player sexually assaulted her at an off-campus party said she wished her fellow students had been warned.

It would have helped them realize “this really can happen to them,” she said, “because I sure didn’t know it could happen to me.”

Campus police argue that in some sexual assault cases, alerts may not be necessary or even appropriate: when victims come forward long after an incident, for example, making a so-called “timely warning” moot, or when a suspect has been identified or arrested and the community faces no imminent threat.

“If you put too many alerts out, the danger is you’re going to inoculate the campus community against the alerts and they won’t pay attention,” said Capt. Frank Belcastro, of San Jose State’s University Police Department.

But some rape-prevention advocates argue the silence contributes to a false sense that rape rarely happens. They say that the tendency by colleges to issue alerts only when unknown suspects are involved perpetuates the myth that only strangers commit sexual assault, when the vast majority are committed by acquaintances.

At Stanford, where “the grass is manicured and the sun is shining,” said graduate student Emma Tsurkov, students need to know the everyday risks lurking where they least expect them. “It creates an illusion for students that it’s safe to go to parties here,” she said, “when we know it’s not the case.”

Stanford did not issue an alert about the now-infamous incident involving Brock Turner, the former swimmer convicted by a jury of sexually assaulting an unconscious woman outside a Stanford fraternity party in January 2015. Turner was immediately apprehended and arrested, factors that police say tend to lessen the general threat to campus.

“The ongoing threat is mitigated once we know who the person is,” San Jose State’s Belcastro said.

The law does not permit colleges to take into account victims’ wishes when deciding whether to issue an alert, said Bill Larson, a spokesman for Stanford’s campus police department. But, he said, many have begged the department not to do so, fearing a “media frenzy” or more public scrutiny.

“The seemingly simple act of notifying the community is quite upsetting to many victims,” Larson wrote in an email.

UC Berkeley, which has been under intense scrutiny in recent years for its handling of sexual assault cases, has become more open about the problem, McConnell said. The campus issued about a dozen warnings last year, including sexual assaults reported at fraternity parties and in a dorm room by a co-worker.

On Monday evening, Berkeley students’ phones pinged with back-to-back text messages about sexual assaults reported at fraternity parties over the weekend. On Tuesday, the councils for the campuses’ fraternities and sororities condemned the alleged attacks as “vile” and announced they would voluntarily suspend social activities “until we can re-evaluate our risk management practices and care for those who have been affected.”

If the alert hadn’t made the reports public, McConnell said, people probably would not be talking about the problem.

“It can’t go ignored if you’re constantly getting alerts in your email,” she said. “I think that silence is just a way of perpetuating the problem.”